In this study, the effects of an oxidation catalytic converter (OCC) on regulated and unregulated emissions from a 1991 prototype Cummins I.10-310 diesel engine fueled with a 0.01 weight percent sulfur fuel were investigated. The OCC's effects were determined by measuring and comparing selected raw exhaust emissions with and without the platinum-based OCC installed in the exhaust system, with the engine operated at three steady-state modes.It was found that the OCC had no significant effect on oxides of nitrogen (NOX) and nitric oxide (NO) at any mode, but reduced hydrocarbon (HC) emmissions by 60 to 70 percent. The OCC reduced total particulate matter (TPM) levels by 27 to 54 percent, primarily resulting from 53 to 71 percent reductions of the soluble organic fraction (SOF). The OCC increased sulfate (SO42-) levels at two of the three modes (modes 9 and 10), but the overall SO42- contribution to TPM was less than 6 percent at all modes due to the low sulfur level of the fuel. The solids fraction (SOL) was not significantly affected by the OCC at any mode. Vapor phase organics (termed XOC) were decreased by 56 to 68 percent. The particle size distributions with the OCC were similar to those under baseline conditions, with virtually no change in either the nuclei or the accumulation mode.The OCC generally reduced the measured polynuclear aromatic hydrocarbons (PAH) associated with the SOF and XOC. Some nitro-PAH levels (e.g., particle-associated 1,6- DNP) were lower when the OCC was used, but due to higher variability in the data (compared to the PAH data), statistical - based comparisons were not conducted. The OCC generally increased the mutagenic activity per mass of the SOF and XOC; however, for all but one case, these increases were offset by the decreases in the SOF and XOC levels.