The effect of diesel fuel properties and composition on regulated emissions has been investigated in an IDI naturally aspirated passenger car equipped with an oxidation catalyst. The influence of diesel fuel changes on emissions from this same vehicle without the catalyst have been reported in a previous SAE paper (1).*The addition of the catalyst to this ‘clean’ car further reduced emissions, especially those of hydrocarbons and carbon monoxide. Particulate emissions were reduced to below the proposed 1996 European limit of 0.08 g/km. The catalyst was especially effective in reducing particulates from the higher density fuels, but had no influence on NOx. The catalyst was ‘sulphur tolerant’; changes in fuel sulphur content between 0.01 and 0.2% wt sulphur had a an insignificant effect on particulate emissions.Variations in fuel properties produced a significant influence on emissions, although the effect was less in this car, with a catalyst, than in the non-catalyst version. Tests on 13 fuels were designed to investigate the influence on emissions of total aromatics, cetane, heavy end distillation (T95), density and sulphur. Of these, the fuel property which correlated best with particulates was density; T95 had a very minor influence, sulphur had an insignificantly small influence, and total aromatics content had no significant effect.These effects were in line with those observed for the non-catalyst version of this vehicle.Changes in Cetane quality (in the range 48 to 62) had no effect on particulates in the non-catalyst car for fuels with densities within the calibration range of the engine (1). However, in this study, the lower levels of particulates produced by the catalyst equipped vehicle were found to be influenced to a minor, but statistically significant extent, by cetane. In this engine and catalyst configuration, increasing the cetane number was associated with a very small increase in particulates.Although total aromatics were shown to have no influence on particulates, for the fuels used in this study, there is a high correlation between density and di/triaromatics. Hence there is a possibility that di/triaromatics may have an influence on particulates. This is the subject of an ongoing study designed to separately estimate the effects of density and multi-ring aromatics.