In response to strong environmental concerns, considerable effort has been directed at identifying actual emissions resulting from light natural gas vehicles operation by the Swiss Federal Institute of Technology in Lausanne. Claims are frequently made regarding the emissions of CNG systems in catalysed vehicles and their performance over legislated cylces. This paper investigates the after catalyser emissions and the effectiveness of catalysers designed for petrol operation in a Renault Express 1.4 liter converted to CNG. A comparison is made between the American FTP75 cycle and the European 91/441/EEjC cycle, including the effect of cold temperatures on emissions, both on petrol and CNG operation. Based on the results, a accounting for the effect of ambient temperature. Emission measurements include separate consideration of methane and non methane hydrocarbons.