Emissions from heavy-duty vehicles may be reduced through the introduction of clean diesel formulations, and through the use of catalyzed particulate matter filters that can enjoy increased longevity and performance if ultra-low sulfur diesel is used. Twenty over-the-road tractors with Detroit Diesel Series 60 engines were selected for this study. Five trucks were operated on California (CA) specification diesel (CARB), five were operated on ARCO (now BP Amoco) EC diesel (ECD), five were operated on ARCO ECD with a Johnson-Matthey Continuously Regenerating Technology (CRT) filter and five were operated on ARCO ECD with an Engelhard Diesel Particulate Filter (DPX). The truck emissions were characterized using a transportable chassis dynamometer, full-scale dilution tunnel, research grade gas analyzers and filters for particulate matter (PM) mass collection. Two test schedules, the 5 mile route and the city-suburban (heavy vehicle) route (CSR), were employed. The ARCO ECD offered reductions (with and without PM aftertreatment) in oxides of nitrogen (NOX) on the order of 15 to 20% relative to CA diesel for both test schedules, although the reductions were not statistically significant (at the 95% confidence level) owing to high vehicle-to-vehicle variability. ECD produced no reduction in PM emissions over CA diesel without aftertreatment, but the PM mass was virtually eliminated on both cycles when ECD was used in conjunction with the CRT and DPX. The aftertreatment devices also served to remove the bulk of carbon monoxide (CO) and hydrocarbon (HC) in the tailpipe emissions. It is concluded that a strategy that employs PM filters with clean diesel may reduce NOx and eliminate most of the mass of other regulated emissions through oxidation. A companion paper, SAE Technical Paper 2000-01-2821, reports on the fleet selection, truck retrofits, operation and cost for these same Class 8 trucks.