An experimental study was undertaken to investigate emissions of hydrocarbons, oxides of nitrogen, carbon monoxide, and methane hydrocarbons emitted by natural gas fueled engines and the extent of their conversion in catalysts. Two engines were used in the study: a four cylinder, 1.6 liter, spark ignition engine and a modified version of the same engine with only one of the cylinders operating at 0.4 liter capacity. Two-way and three-way catalysts were used to treat exhaust gases leaving the engine. Natural gas was supplied through gas carburetors operated at regulated pressures and supplying air-fuel ratios in the desired range.The results of the investigation showed that oxides of nitrogen could not be reduced in a three-way catalyst to the levels found in gasoline fueled engines when the operating air-fuel ratio was stoichiometric. This was believed to be due to lower levels of carbon monoxide emitted by natural gas fueled engines at stoichiometric condition relative to a gasoline fueled engine at the same equivalence ratio. This was further confirmed by injecting small quantities of carbon monoxide in engine exhaust upstream of the converter. It was decided to operate the engines at slightly richer conditions to generate additional carbon monoxide in the exhaust. Measurements of chemical species before and after the converter showed substantial improvements in converter efficiency when the engines were operated at slightly richer air-fuel ratio and when the exhaust gas temperatures were over 750° K. Furthermore, methane conversion efficiencies in the three-way converter were in excess of 60% depending on exhaust gas temperatures. In the single cylinder engine tests lower exhaust gas temperatures yielded low methane conversion efficiencies than in the four cylinder engine case.