I couldn't find any information on whether catchSTM catches asynchronous exceptions so I tried to run the following: import Control.Concurrent.STM import Control.Concurrent import Control.Exception import Prelude hiding (catch) test = do tid <- myThreadId forkIO (threadDelay 5000000 >> throwTo tid (AssertionFailed "Exception in forked thread!")) (atomically $ retry `catchSTM` stmHandler) `catch` (ioHandler tid) where stmHandler (e::SomeException) = throw $ AssertionFailed ("Caught Exc. in STM; Rethrowing exception: "++ show e) ioHandler tid (e::SomeException) = print (tid,"Caught Exception in IO: ",e) Yielding the following output: # (ThreadId 6942,"Caught Exception in IO: ",Exception in forked thread!) Apparently the exception is caught by "catch" and not by "catchSTM". So the point is that catchSTM is only meant to be used for non-async exceptions, right? Regards, Peter