diff --git a/calendar/base/content/calendar-common-sets.js b/calendar/base/content/calendar-common-sets.js
index cff74451c59..432cdc8702d 100644
--- a/calendar/base/content/calendar-common-sets.js
+++ b/calendar/base/content/calendar-common-sets.js
@@ -95,13 +95,13 @@ var calendarController = {
     isCommandEnabled: function cC_isCommandEnabled(aCommand) {
         switch (aCommand) {
             case "calendar_new_event_command":
-                return this.writable;
+                return this.writable && this.calendars_support_events;
             case "calendar_modify_event_command":
                 return this.item_selected;
             case "calendar_delete_event_command":
                 return this.selected_items_writable;
             case "calendar_new_todo_command":
-                return this.writable;
+                return this.writable && this.calendars_support_tasks;
             case "calendar_delete_todo_comand":
                 return this.writable; // XXX are selected todo items readonly?
 
@@ -350,6 +350,32 @@ var calendarController = {
                this.item_selected &&
                !this.selected_events_readonly &&
                (!this.offline || !this.selected_events_requires_network);
+    },
+
+    get calendars_support_tasks() {
+        // XXX We might want to cache this
+        var calendars = getCalendarManager().getCalendars({});
+
+        for each (var cal in calendars) {
+            if (isCalendarWritable(cal) &&
+                cal.getProperty("capabilities.tasks.supported") !== false) {
+                return true;
+            }
+        }
+        return false;
+    },
+
+    get calendars_support_events() {
+        // XXX We might want to cache this
+        var calendars = getCalendarManager().getCalendars({});
+
+        for each (var cal in calendars) {
+            if (isCalendarWritable(cal) &&
+                cal.getProperty("capabilities.events.supported") !== false) {
+                return true;
+            }
+        }
+        return false;
     }
 };
 
diff --git a/calendar/base/content/calendar-item-editing.js b/calendar/base/content/calendar-item-editing.js
index 978dd060898..8cf0d27a12b 100644
--- a/calendar/base/content/calendar-item-editing.js
+++ b/calendar/base/content/calendar-item-editing.js
@@ -1,4 +1,3 @@
-/* -*- Mode: javascript; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
 /* ***** BEGIN LICENSE BLOCK *****
  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
  *
@@ -186,14 +185,30 @@ function openEventDialog(calendarItem, calendar, mode, callback, job)
     mode = mode || "new";
     calendar = calendar || getSelectedCalendar();
     var calendars = getCalendarManager().getCalendars({});
-    calendars = calendars.filter(function(el) { return !el.readOnly; });
+    calendars = calendars.filter(isCalendarWritable);
 
-    if (calendar.readOnly && mode == "new" && calendars.length < 1) {
-        // All calendars are marked readonly, don't show the dialog
+    var isItemSupported;
+    if (isToDo(calendarItem)) {
+        isItemSupported = function isTodoSupported(cal) {
+            return (cal.getProperty("capabilities.tasks.supported") !== false);
+        };
+    } else if (isEvent(calendarItem)) {
+        isItemSupported = function isEventSupported(cal) {
+            return (cal.getProperty("capabilities.events.supported") !== false);
+        };
+    }
+
+    // Filter out calendars that don't support the given calendar item
+    calendars = calendars.filter(isItemSupported);
+
+    if (mode == "new" && calendars.length < 1 &&
+        (!isCalendarWritable(calendar) || !isItemSupported(calendar))) {
+        // There are no writable calendars or no calendar supports the given
+        // item. Don't show the dialog.
         return;
-    } else if (calendar.readOnly && mode == "new") {
-        // If the default calendar is marked readOnly, pick the first
-        // non-readOnly calendar
+    } else if (mode == "new" &&
+               (!isCalendarWritable(calendar) || !isItemSupported(calendar))) {
+        // Pick the first calendar that supports the item and is writable
         calendar = calendars[0];
         if (calendarItem) {
             // XXX The dialog currently uses the items calendar as a first
@@ -230,7 +245,7 @@ function openEventDialog(calendarItem, calendar, mode, callback, job)
 
     // open the dialog modeless
     var url = "chrome://calendar/content/sun-calendar-event-dialog.xul";
-    if (isInvitation || calendar.readOnly) {
+    if (isInvitation || !isCalendarWritable(calendar)) {
         url = "chrome://calendar/content/calendar-summary-dialog.xul";
     }
     openDialog(url, "_blank", "chrome,titlebar,resizable", args);
diff --git a/calendar/base/content/calendar-multiday-view.xml b/calendar/base/content/calendar-multiday-view.xml
index c89992c6743..d7e2e2a611c 100644
--- a/calendar/base/content/calendar-multiday-view.xml
+++ b/calendar/base/content/calendar-multiday-view.xml
@@ -1548,7 +1548,8 @@
         <parameter name="aMouseX"/>
         <parameter name="aMouseY"/>
         <body><![CDATA[
-          if (aOccurrence.calendar.readOnly) {
+          if (!isCalendarWritable(aOccurrence.calendar) ||
+              aOccurrence.calendar.getProperty("capabilities.events.supported") === false) {
               return;
           }
 
@@ -1742,7 +1743,9 @@
         this.calendarView.selectedDay = this.mDate;
 
         // If the selected calendar is readOnly, we don't want any sweeping.
-        if (getSelectedCalendar().readOnly) {
+        var cal = getSelectedCalendar();
+        if (!isCalendarWritable(cal) ||
+            cal.getProperty("capabilities.events.supported") === false) {
             return;
         }
 
diff --git a/calendar/base/public/calICalendar.idl b/calendar/base/public/calICalendar.idl
index c91dc09213b..493770ee043 100644
--- a/calendar/base/public/calICalendar.idl
+++ b/calendar/base/public/calICalendar.idl
@@ -131,6 +131,8 @@ interface calICalendar : nsISupports
    *   capabilities.attachments.supported Supports attachments
    *   capabilities.privacy.supported     Supports a privacy state
    *   capabilities.priority.supported    Supports the priority field
+   *   capabilities.events.supported      Supports tasks
+   *   capabilities.tasks.supported       Supports events
    *
    * The following capabilities are used to restrict the values for specific
    * fields. An array should be specified with the values, the default
diff --git a/calendar/prototypes/wcap/sun-calendar-event-dialog.js b/calendar/prototypes/wcap/sun-calendar-event-dialog.js
index b887b17c0d0..bacba2dccf0 100644
--- a/calendar/prototypes/wcap/sun-calendar-event-dialog.js
+++ b/calendar/prototypes/wcap/sun-calendar-event-dialog.js
@@ -286,6 +286,17 @@ function loadDialog(item) {
 
     loadDateTime(item);
 
+    var isItemSupported;
+    if (isToDo(calendarItem)) {
+        isItemSupported = function isTodoSupported(cal) {
+            return (cal.getProperty("capabilities.tasks.supported") !== false);
+        };
+    } else if (isEvent(calendarItem)) {
+        isItemSupported = function isEventSupported(cal) {
+            return (cal.getProperty("capabilities.events.supported") !== false);
+        };
+    }
+
     // add calendars to the calendar menulist
     var calendarList = document.getElementById("item-calendar");
     var calendars = getCalendarManager().getCalendars({});
@@ -303,7 +314,9 @@ function loadDialog(item) {
                 }
             }
             selectIndex++;
-        } else if (calendar && !calendar.readOnly) {
+        } else if (calendar &&
+                   isCalendarWritable(calendar) &&
+                   isItemSupported(calendar)) {
             var menuitem = calendarList.appendItem(calendar.name, i);
             menuitem.calendar = calendar;
             if (calendarToUse) {
diff --git a/calendar/providers/gdata/components/calGoogleCalendar.js b/calendar/providers/gdata/components/calGoogleCalendar.js
index 3774c52fb00..e7c774110be 100644
--- a/calendar/providers/gdata/components/calGoogleCalendar.js
+++ b/calendar/providers/gdata/components/calGoogleCalendar.js
@@ -172,6 +172,7 @@ calGoogleCalendar.prototype = {
             // Capabilities
             case "capabilities.attachments.supported":
             case "capabilities.priority.supported":
+            case "capabilities.tasks.supported":
                 return false;
             case "capabilities.privacy.values":
                 return ["DEFAULT", "PUBLIC", "PRIVATE"];