The steam reforming of methane using catalytic membrane reactors has been simulated for two types of membranes - a dense Pd/Ag composite membrane and a series of microporous membranes. Various values of hydrogen separation factors were investigated for the latter. In general the Pd/Ag membranes gave the better performance, for all parameters investigated including temperature, pressure, sweep ratio, and membrane thickness. However, the use of steam as a sweep gas produced an improved performance for the microporous membranes operating in the Knudsen region.