A Monte Carlo program has been developed to simulate the gas dynamics inside flow reactors (Knudsen cells) specially adapted to study heterogeneous reactions. Because the reactors are operated under molecular flow conditions, the Monte Carlo technique can be applied to calculate trajectories of individual molecules considering only gas-wall collisions. Ensembles of trajectories are then used to generate synthetic kinetic data as a function of specific reactor geometries (surface-to-volume ratio, injector position, reactive-surface diameter, etc.) and analyzed to yield rate constants. The calculated rate constants thus obtained are useful in assessing the systematic error associated with a rate-constant determination and in the designing of future geometries of Knudsen cells. ©1997 American Institute of Physics.