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The promulgation of the public transit fleet rule by the California Air Resources Board (CARB) in 2000, has given transit fleet operators the option of choosing the alternative fuel path in order to reduce their fleet average NOx and PM emissions. Natural gas being an abundant domestic fuel, has found its way as an economically and technologically feasible alternative fuel option. Many studies have shown the clean burning nature of natural gas with lower NOx and near zero Particulate Matter (PM) emissions from heavy duty natural gas vehicles. Though natural gas fueled vehicles emit lower NOx and PM than their diesel counterparts, the emissions of carbon monoxide (CO) and total hydrocarbons (THC) are higher. This necessitates the use of a suitable exhaust after-treatment device to attain complete emission benefits.;The objective of the study was to measure regulated and unregulated emissions from CNG fueled heavy-duty transit bus with and without the after-treatment device present. The study conducted in Riverside, California utilized two CNG fueled transit buses one from Riverside Transit Authority (RTA) and the other from Los Angeles County Metro Transit Authority (LACMTA). The study required the complete chemical speciation of exhaust from the RTA bus with and without the after-treatment device so as to evaluate the effectiveness of the after-treatment device in reducing both regulated and unregulated emissions. The buses were retrofitted with an oxidation catalytic converter manufactured by Engine Control Systems (ECS). The buses were tested on a heavy duty chassis dynamometer part of the West Virginia University Transportable Heavy Duty Vehicle Emissions Testing Laboratory (WVTHDVETL). The transit buses were exercised over a double length Orange County Transit Authority (OCTA) cycle to characterize its emission levels. The analysis of the unregulated sample, which included Poly Aromatic Hydrocarbons (PAH), aldehydes, Volatile Organic Compounds (VOC), metals and elemental/organic carbon was done by Desert Research Institute (DRI).;The results of the regulated emissions showed a 99% reduction in CO and 62% reduction in THC with the after-treatment device present. The unregulated speciation results showed 96% reduction in carbonyl compounds with formaldehyde being the major contributor, 46% reduction in PAH compounds, 60% reduction in nitro-PAH compounds and 93% reduction in VOC. There was an overall 27% increase in metal content in exhaust with the after-treatment device present. There was no effect on the organic carbon concentration with the after-treatment device present.