diff --git a/CHANGELOG.md b/CHANGELOG.md
index f34a9d1..c3dc275 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - **Breaking:** `AwaitOperationConfiguration` cases are prefixed with `Await` (`AwaitSequential`, `AwaitParallel 4`, ...) so they no longer collide with `System.Threading.Tasks.Parallel`
+- **Breaking:** the `Async` `toLookup` overloads no longer take a `cancellationToken`; they observe the token of the computation
 - **Breaking:** the `OptionExtensions`/`ValueOptionExtensions` types are replaced by `Observable.choose`, which takes a `voption` chooser, and `ObservableOption.choose` for `option`; the `Observable.Extensions` type is replaced by `Observable.ofSeq`
 - **Breaking:** after `open FSharp.Control.R3`, `Observable.choose` is the R3 function, like `Observable.map` and `Observable.filter` already were; use `Microsoft.FSharp.Control.Observable.choose` for `IObservable` and F# events
 - **Breaking:** after `open FSharp.Control.R3`, the bare type name `Observable` refers to `FSharp.Control.R3.ObservableFactories.Observable`; write `open type R3.Observable` and extend `R3.Observable` by its full name
@@ -28,6 +29,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - `rxquery` emitted its elements on the thread pool, out of order and after the source had moved on; `yield` and `zero` are now synchronous
 - `rxquery` `sumBy` passed `null` to the `(+)` of reference types
+- A positional comparer or element selector passed to the `Async` `toLookup` was silently ignored
 - `chunkBy` accepted a non-positive window length of `ChunkTimeSpanCount` and `ChunkMillisecondsCount`, which failed every element
 - Misleading XML docs of `catch`
 
diff --git a/src/FSharp.Control.R3/AsyncObservable.fs b/src/FSharp.Control.R3/AsyncObservable.fs
index cb25a62..506142e 100644
--- a/src/FSharp.Control.R3/AsyncObservable.fs
+++ b/src/FSharp.Control.R3/AsyncObservable.fs
@@ -115,37 +115,39 @@ module Observable =
 [<AutoOpen>]
 module Extensions =
 
-    open System.Runtime.CompilerServices
-    open System.Runtime.InteropServices
+    open System.Collections.Generic
 
-    [<AbstractClass; Sealed; Extension>]
+    // The overloads take no cancellation token: like every other Async function they observe the token of the computation.
+    // An unused generic [<Optional>] token parameter used to swallow a positional keyComparer or elementSelector.
+    [<AbstractClass; Sealed>]
     type Observable private () =
 
-        static member toLookup (source, keySelector : 'T -> 'Key, [<Optional>] cancellationToken) = async {
+        static member toLookup (source : Observable<'T>, keySelector : 'T -> 'Key) = async {
             let! ct = Async.CancellationToken
             return!
                 ObservableExtensions.ToLookupAsync (source, keySelector, ct)
                 |> Async.AwaitTask
         }
 
-        static member toLookup (source, keySelector : 'T -> 'Key, keyComparer, [<Optional>] cancellationToken) = async {
+        static member toLookup (source : Observable<'T>, keySelector : 'T -> 'Key, keyComparer : IEqualityComparer<'Key>) = async {
             let! ct = Async.CancellationToken
             return!
                 ObservableExtensions.ToLookupAsync (source, keySelector, keyComparer = keyComparer, cancellationToken = ct)
                 |> Async.AwaitTask
         }
 
-        static member toLookup (source, keySelector : 'T -> 'Key, elementSelector : 'T -> 'Element, [<Optional>] cancellationToken) = async {
+        static member toLookup (source : Observable<'T>, keySelector : 'T -> 'Key, elementSelector : 'T -> 'Element) = async {
             let! ct = Async.CancellationToken
             return!
                 ObservableExtensions.ToLookupAsync (source, keySelector, elementSelector = elementSelector, cancellationToken = ct)
                 |> Async.AwaitTask
         }
 
-        static member toLookup (source, keySelector : 'T -> 'Key, elementSelector : 'T -> 'Element, keyComparer, [<Optional>] cancellationToken) =
-            async {
-                let! ct = Async.CancellationToken
-                return!
-                    ObservableExtensions.ToLookupAsync (source, keySelector, elementSelector, keyComparer = keyComparer, cancellationToken = ct)
-                    |> Async.AwaitTask
-            }
+        static member toLookup
+            (source : Observable<'T>, keySelector : 'T -> 'Key, elementSelector : 'T -> 'Element, keyComparer : IEqualityComparer<'Key>)
+            = async {
+            let! ct = Async.CancellationToken
+            return!
+                ObservableExtensions.ToLookupAsync (source, keySelector, elementSelector, keyComparer = keyComparer, cancellationToken = ct)
+                |> Async.AwaitTask
+        }