Different selective catalytic reduction (SCR) and oxidation catalyst systems were assessed to study the effects of these catalysts on the NOx, the hydrocarbon and the particle emissions from natural gas engines. High NOx reductions were observed at low ammonia slips. A separate oxidation catalyst was used downstream of the SCR. The NOx reduction efficiency was found to depend on the exhaust gas temperature. The efficiency of a catalyst reactor to remove hydrocarbons was found to depend on the hydrocarbon species, the exhaust temperature as well as the exhaust flow. The total measured PM mass was lower downstream of the catalyst than in the engine exhaust at all temperatures studied. Catalyst systems were found to have significant effects on particulate emissions.