Test were undertaken with a Renault Express 1.4 litre converted to natural gas operation. The effect of cold starts at cold temperatures and vehicle weight on taile pipe emissions were investigated with petrol and natural gas operation over the FTP75 and 91/441/EEC drive cycles. The results show that the emissions with natural gas are unaffected by cold temperature, unlike petrol emission which are several times higher at -15°C that at 25°C. A crude simulation, accounting for the actual temperature, shows that the conversion of a significant quantity of light duty vehicles to natural gas operation could reduce the emissions of CO and HC by more that 90% in Switzerland.