A study was performed in the spring of 2001 to chemically characterize exhaust emissions from trucks and buses fueled by various test fuels and operated with and without diesel particle filters. This study was part of a multi-year technology validation program designed to evaluate the emissions impact of ultra-low sulfur diesel fuels and passive diesel particle filters (DPF) in several different heavy-duty vehicle fleets operating in Southern California. The overall study of exhaust chemical composition included organic compounds, inorganic ions, individual elements, and particulate matter in various size-cuts. Detailed descriptions of the overall technology validation program and chemical speciation methodology have been provided in previous SAE publications (2002-01-0432 and 2002-01-0433).This paper focuses on the identification and quantification of organic compounds in vehicle exhaust, including: Non-Methane Hydrocarbons (NMHC), Carbonyl compounds, polycyclic aromatic hydrocarbons (PAH) - including nitro-PAHs (n-PAH) - and polychlorodibenzo dioxins and furans. Testing was conducted on four diesel vehicles and two standard compressed natural gas (CNG) vehicles. The diesel vehicles were tested using a representative California diesel, two ultra low sulfur diesel fuels - ECD-1 and ECD - along with Fischer-Tropsch diesel, with and without passive catalyzed particle filters. The gas fueled vehicles were tested using motor vehicle-grade CNG with no after-treatment. The data presented in this paper illustrate that ultra low sulfur diesel (ULSD) in combination with diesel particle filters (DPFs) result in emission reductions of 50-90% when compared to CARB diesel. In addition, the DPFs tested are shown not to merely lower the levels of semi-volatiles and particulate bound species, but to reduce NMHC and Carbonyl emissions as well. This might be due to the oxidative properties of the DPFs tested. For transit buses, the total emissions for diesel vs. CNG fueled vehicles tested as comparable when ULSD+DPF was used. However, CNG vehicles did exhibit higher levels of NMHCs and Carbonyl compounds.