An experimental program was conducted to evaluate the contribution of fuel sulfur to brake specific particulate emissions. Three diesel engines - typical for medium and heavy duty truck applications - were tested in four transient cycles of various average loading factors. Three experimental fuels were used having the same basic characteristics, but a sulfur content of 0.05%, 0.19%, and 0.29%, respectively.Analysis of test results concluded that, among the factors investigated, fuel sulfur was the most important factor contributing to brake specific particulate sulfate variation. For an increase of 0.1% in fuel sulfur, brake specific particulates increase by about 0.025 g/bhp-hr, due to addition of soluble sulfates and bound water. Combustion system, engine type, cycle loading, particulate makeup, had only a weak contribution to the brake specific sulfate variation.Conversion rates of fuel sulfur to sulfates on particulates were in the range of one to three percent. These results were computed from actual test results and were confirmed by regression analysis of the experimental data.