When designing a cryopump, it is desirable to have some method of predicting cryopump performance for the different cryopanel geometries that are being considered. Monte Carlo simulation is a method that has been used for evaluating the pumping speed of a;cryopump when it has not yet accumulated any gas. The simulation of hydrogen pumping has its own challenges due to the typical location of the hydrogen pumping sites. As gases accumulate on the pumping surfaces, the passageways for hydrogen gas are restricted. In this article we describe the work that was done, using Monte Carlo simulation, to study the hydrogen pumping speed and capacity of a 200 mm cryopump as it accumulates hydrogen, water and argon. The hydrogen pumping speed is predicted as a function of the amount of other gases that have been accumulated.