Engine fuel tests were conducted with two oxygenates blended with conventional diesel and a synthetic Fisher-Tropsch (F-T) diesel to determine their emissions reduction potential. The oxygenated additives evaluated were dimethoxy methane (DMM) (also known as methylal) and diethyl ether (DEE). Blends of 5, 10, 20 and 30% by volume were investigated. The test engine was a 1993 Cummins B5.9 diesel, and data was collected for steady state operation at nine engine speed-load conditions.Experimental results show that all of the test fuels reduce PM when data is averaged across the nine engine operating modes. The largest reductions in PM were observed with a blend of 30% DMM in diesel, which yielded a 35% reduction compared to the baseline diesel fuel. Lower DMM blend levels also resulted in PM reductions, but to a lesser extent. On a modal averaged basis, F-T diesel reduced PM emissions by 29%, and DEE in concentrations of 10 to 30% reduced PM emissions by between 13 and 24%.On a modal averaged basis, NOx emissions for F-T diesel and the DMM and DEE blends were reduced by between 1 and 10%. Corresponding increases in fuel consumption (beyond that necessary to compensate for the differences in energy densities) were also observed. Modal averaged emissions of THC and CO were in most cases higher for the test fuels, but their overall levels remained low.Investigation of individual modal data revealed large differences in the effect of the DMM and DEE blends on PM emissions. In general, measured PM levels were drastically (as much as 76%) lower for the higher engine power test modes. For the lower power modes, however, PM emissions were often unchanged or even increased.