We describe a general technique for converting an online algorithm B to a truthtelling mechanism. We require that the original online competitive algorithm has certain "niceness" properties in that actions on future requests are independent of the actual value of requests which were accepted (though these actions will of course depend upon the set of accepted requests). Under these conditions, we are able to give an online truthtelling mechanism (where the values of requests are given by bids which may not accurately represent the valuation of the requesters) such that our total profit is within O( + log ?) of the optimum offline profit obtained by an omniscient algorithm (one which knows the true valuations of the users). Here is the competitive ratio of B for the optimization version of the problem, and ? is the ratio of the maximum to minimum valuation for a request. In general there is an (log ?) lower bound on the ratio of worst-case profit for a truthtelling mechanism...