Hi, I faced some strange behavior while using timers (boost_1_38_0/boost_1_39_0). Here is an easy way to reproduce such behavior (note: I use async timeouts and a single thread): =experiment 1= Timer1 expires after 2 seconds and reschedules itself to fire again 2 seconds later. Timer2 expires after 2 seconds and cancels Timer1. Result: Timer2's handler gets called but the cancel call on Timer1 returns 0. Timer1's handler gets called (although it should have been cancelled) and therefore, Timer1 repeats itself indefinitely. =experiment 2= Timer1 expires after 1995 milliseconds and primes itself to fire again 1995 milliseconds later. Timer2 expires after 2 seconds and cancels Timer1. Result: Timer1's handler gets called and reschedules itself. Timer2's handler gets called and Timer1 gets canceled properly. =experiment 3= Timer1 expires after 2005 milliseconds and primes itself to fire again 2005 milliseconds later. Timer2 expires after 2 seconds and cancels Timer1. Result: Timer2's handler gets called and Timer1 gets canceled properly. =experiment 4= Repeat experiment 1 but change the expiration time of Timer1 instead of canceling it from Timer2's handler. Result: Timer2's handler gets called and Timer1 gets rescheduled properly. Note that Timer1's handler doesn't get called at all before the new expiry time although it should be called with an operation_aborted error code. =summary= When Timer1 and Timer2 expire at a very close time, Timer2 cannot cancel Timer1 although it can reschedule it. Is it an expected behavior? Is it due to the design of Asio? Regards, Laurent