A major part of urban traffic in many European cities is due to diesel-powered vehicles, which occasionally, albeit illegally, run on heating gas oil rather than the more expensive road diesel; this practice is no doubt abetted by the fact that the properties of the two fuels are quite similar. The purpose of the work described in this paper was to look into the possibility of altering the properties of heating oil so as to make it less desirable as a diesel fuel but without adverse environmental effects. Tests were run with two diesel-powered vehicles and a domestic boiler and included measurements of emissions of smoke, carbon monoxide, unburnt hydrocarbons, nitrogen oxides, and boiler thermal efficiency. Smoke opacity of diesel exhaust was found to have an almost linear relationship with many of the basic interrelated properties (cetane number, viscosity, density) of fuels with similar distillation characteristics, whereas the presence of heavier components tended to increase smoke. Addition of aromatic oxygenates (cresols) reduced the cetane number of diesel fuel but at the same time reduced smoke and NOx in diesel exhaust; the same materials appeared to enhance the thermal efficiency of the boiler and lower its smoke emissions, while slightly increasing the emitted NO.