Tests 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 tail pipe emissions were investigated with petrol and natural gas operation over the FTP75 and the 91/441/EEC drive cycles. The results show that the emissions with natural gas are unaffected by cold temperature, unlike petrol emissions which are several times higher at -15°C than at 25°C. A crude simulation, accounting for the actual temperature shows that the conversion of a significant quantity of light duty vehicle to natural gas operation could reduce the emissions of CO and HC by more that 90% in Switzerland.