diff --git a/Riot/Analytics/Analytics.m b/Riot/Analytics/Analytics.m
index a376fbef8..5ba48b694 100644
--- a/Riot/Analytics/Analytics.m
+++ b/Riot/Analytics/Analytics.m
@@ -28,7 +28,6 @@ NSString *const kAnalyticsMetricsActionPattern = @"iOS.%@";
 // E2E telemetry is stored under a Piwik category called "E2E".
 NSString *const kAnalyticsE2eCategory = @"E2E";
 NSString *const kAnalyticsE2eDecryptionFailureAction = @"Decryption failure";
-NSString *const kAnalyticsE2eDecryptionFailureReasonNoReason = @"no_reason";
 
 
 @import PiwikTracker;
@@ -174,13 +173,16 @@ NSString *const kAnalyticsE2eDecryptionFailureReasonNoReason = @"no_reason";
 
 #pragma mark - MXDecryptionFailureDelegate
 
-- (void)trackFailures:(NSUInteger)failuresCount
+- (void)trackFailures:(NSDictionary<NSString *,NSNumber *> *)failuresCounts
 {
-    [[PiwikTracker shared] trackWithEventWithCategory:kAnalyticsE2eCategory
-                                               action:kAnalyticsE2eDecryptionFailureAction
-                                                 name:kAnalyticsE2eDecryptionFailureReasonNoReason
-                                               number:@(failuresCount)
-                                                  url:nil];
+    for (NSString *reason in failuresCounts)
+    {
+        [[PiwikTracker shared] trackWithEventWithCategory:kAnalyticsE2eCategory
+                                                   action:kAnalyticsE2eDecryptionFailureAction
+                                                     name:reason
+                                                   number:failuresCounts[reason]
+                                                      url:nil];
+    }
 }
 
 @end
diff --git a/Riot/Analytics/DecryptionFailure.h b/Riot/Analytics/DecryptionFailure.h
index df9f1af75..7ea43962c 100644
--- a/Riot/Analytics/DecryptionFailure.h
+++ b/Riot/Analytics/DecryptionFailure.h
@@ -16,6 +16,18 @@
 
 #import <Foundation/Foundation.h>
 
+/**
+ Failure reasons as defined in https://docs.google.com/document/d/1es7cTCeJEXXfRCTRgZerAM2Wg5ZerHjvlpfTW-gsOfI.
+ */
+struct DecryptionFailureReasonStruct
+{
+    __unsafe_unretained NSString * const unspecified;
+    __unsafe_unretained NSString * const olmKeysNotSent;
+    __unsafe_unretained NSString * const olmIndexError;
+    __unsafe_unretained NSString * const unexpected;
+};
+extern const struct DecryptionFailureReasonStruct DecryptionFailureReason;
+
 /**
  `DecryptionFailure` represents a decryption failure.
  */
diff --git a/Riot/Analytics/DecryptionFailure.m b/Riot/Analytics/DecryptionFailure.m
index 7cb470e8c..d43b0ec9f 100644
--- a/Riot/Analytics/DecryptionFailure.m
+++ b/Riot/Analytics/DecryptionFailure.m
@@ -16,6 +16,13 @@
 
 #import "DecryptionFailure.h"
 
+const struct DecryptionFailureReasonStruct DecryptionFailureReason = {
+    .unspecified = @"unspecified_error",
+    .olmKeysNotSent = @"olm_keys_not_sent_error",
+    .olmIndexError = @"olm_index_error",
+    .unexpected = @"unexpected_error"
+};
+
 @implementation DecryptionFailure
 
 - (instancetype)init
diff --git a/Riot/Analytics/DecryptionFailureTracker.h b/Riot/Analytics/DecryptionFailureTracker.h
index 92c7587f1..45a8e1cee 100644
--- a/Riot/Analytics/DecryptionFailureTracker.h
+++ b/Riot/Analytics/DecryptionFailureTracker.h
@@ -16,6 +16,8 @@
 
 #import <Foundation/Foundation.h>
 
+#import "DecryptionFailure.h"
+
 @import MatrixSDK;
 
 @protocol MXDecryptionFailureDelegate;
@@ -62,8 +64,8 @@
 /**
  Stats for decryption failures.
 
- @param failuresCount the number of decryption failures.
+ @param failuresCounts the number of errors per failure reason.
  */
-- (void)trackFailures:(NSUInteger)failuresCount;
+- (void)trackFailures:(NSDictionary<NSString*, NSNumber*> *)failuresCounts;
 
 @end
diff --git a/Riot/Analytics/DecryptionFailureTracker.m b/Riot/Analytics/DecryptionFailureTracker.m
index 240f0ecd2..ed87dd952 100644
--- a/Riot/Analytics/DecryptionFailureTracker.m
+++ b/Riot/Analytics/DecryptionFailureTracker.m
@@ -16,8 +16,6 @@
 
 #import "DecryptionFailureTracker.h"
 
-#import "DecryptionFailure.h"
-
 
 // Call `checkFailures` every `CHECK_INTERVAL`
 #define CHECK_INTERVAL 5
@@ -93,8 +91,26 @@
     DecryptionFailure *decryptionFailure = [[DecryptionFailure alloc] init];
     decryptionFailure.failedEventId = event.eventId;
 
-    // TODO: Need to sync with all platforms
-    // decryptionFailure.reason =;
+    // Categorise the error
+    switch (event.decryptionError.code)
+    {
+        case MXDecryptingErrorUnknownInboundSessionIdCode:
+            decryptionFailure.reason = DecryptionFailureReason.olmKeysNotSent;
+            break;
+
+        case MXDecryptingErrorOlmCode:
+            decryptionFailure.reason = DecryptionFailureReason.olmIndexError;
+            break;
+
+        case MXDecryptingErrorEncryptionNotEnabledCode:
+        case MXDecryptingErrorUnableToDecryptCode:
+            decryptionFailure.reason = DecryptionFailureReason.unexpected;
+            break;
+
+        default:
+            decryptionFailure.reason = DecryptionFailureReason.unspecified;
+            break;
+    }
 
     reportedFailures[event.eventId] = decryptionFailure;
 }
@@ -128,8 +144,16 @@
 
     if (_delegate && failuresToTrack.count)
     {
-        NSLog(@"[DecryptionFailureTracker] trackFailures: %@", @(failuresToTrack.count));
-        [_delegate trackFailures:failuresToTrack.count];
+        // Sort failures by error reason
+        NSMutableDictionary<NSString*, NSNumber*> *failuresCounts = [NSMutableDictionary dictionary];
+        for (DecryptionFailure *failure in failuresToTrack)
+        {
+            failuresCounts[failure.reason] = @(failuresCounts[failure.reason].unsignedIntegerValue + 1);
+        }
+
+        NSLog(@"[DecryptionFailureTracker] trackFailures: %@", failuresCounts);
+
+        [_delegate trackFailures:failuresCounts];
     }
 }