Improvements in diesel engine design to reduce particulate emissions levels, and a recent Environmental Protection Agency (EPA) ruling limiting the maximum sulfur content in diesel fuel, enhanced the viability of catalytic aftertreatment for this market. The Department of Emissions Research, Southwest Research Institute (SwRI), under contract from the Engine Manufacturers Association, (EMA), conducted a search to identify flow-through catalyst technologies available to reduce particulate emissions without trapping. The search revealed a variety of catalyst formulations, washcoats, and substrate designs which were screened on a light-duty diesel. Based on the performance of eighteen converters evaluated, several designs were selected to continue experimentation on a modern technology heavy-duty diesel engine. Test results from this project yielded a better understanding of the mechanisms involved in the reduction of the organic fraction of the particulate, as well as sulfate formation, storage, and release.