ZSM-5 catalysts without (H) and with added metals (Ga, PtGa, Pt) have been studied for propane conversion and aromatization at 530 degrees C, with the contact time varied by three orders of magnitude. From initial rate and selectivity determinations, the role of the modifying metals has been ascertained. On metal-free HZSM-5 propane yields equimolar methane and ethylene as well as propylene. Adding metals increases the rate of propylene formation, as a result of their dehydrogenation activity, which acts additively rather than synergistically. The intrinsic dehydrogenation activity, observed at low conversion and time on stream, increases in the order Ga < PtGa < Pt. At higher conversion and/or longer time on stream, the reverse order is found for the formation of aromatics. It is shown that Pt but not Ga undergoes rapid coke deactivation under aromatization conditions.