Automotive gas oil (AGO) quality affects the emissions from diesel engines, but is not considered to be as influential as engine design or maintenance. Sulphur, density and cetane number are the fuel properties having the greatest influence on emissions. Refineries within the EC are already investing heavily to reduce sulphur levels to 0.05 percent mass by 1996 which will significantly assist the manufacturers of heavy duty diesel engines to meet future emissions standards. Further improvements in fuel quality are being called for by the motor industry which could impact on fuel supply and will be increasingly costly to achieve both financially and in terms of refinery CO2 emissions. The balance between engine emission benefits and environmental and societal costs deteriorates rapidly at extreme fuel qualities. Future AGO specifications should be technically justifiable and based on meeting environmental air quality standards in a cost effective way. The optimum will reflect a balance between equipment and fuel developments. For the covering abstract see IRRD 873243.