Natural gas powered vehicles are attractive in certain applications due to their lower emissions in general than conventional diesel engines and the low cost of natural gas. For stoichiometric natural gas engines, the aftertreatment system typically consists only of a three-way catalyst (TWC). However, increasingly stringent NOx and methane regulations challenge current TWC technologies. In this work, a catalyst reactor system with variable lean/rich switching capability was developed for evaluating TWCs for stoichiometric natural gas engines. The effect of varying frequency and duty-cycle during lean/rich gas switching experiments was measured with a hot-wire anemometer (HWA) due to its high sensitivity to gas thermal properties. A theoretical reactor gas dispersion model was then developed and validated with the HWA measurements. The model is capable of predicting the actual lean/rich gas exposure to the TWC under different testing conditions. Finally, three platinum group metal (PGM) based aftertreatment catalysts with significant differences in PGM loading, Pt/Pd ratio, and oxygen storage capacity were evaluated under simulated stoichiometric natural gas engine exhaust conditions. The results for the three catalysts are compared and discussed in terms of their formulation differences and exposed lean/rich gas environments.