Regulated and volatile organic exhaust species were characterized from a 1993 Chevrolet Lumina operating on CNG, LPG, and reformulated gasoline. For the gaseous fuels, aftermarket conversion kits were installed on the vehicle, and the resulting exhaust emissions were compared to the gasoline baseline results. For all of the fuels, the vehicle was operated over the chassis dynamometer portion of the Federal Test Procedure for light-duty vehicles at fuel/air equivalence ratios of 0.8, 1.0, and 1.2; and exhaust emissions were sampled both with and without the catalytic converter in place. Analyses of exhaust samples included determination of regulated exhaust emissions by CFR methods, hydrocarbon speciation and aldehyde and ketone analyses according to Auto/Oil Phase II methods, and the determination of trace exhaust species by mass spectral analysis methods. Speciation data showed greater than 87 percent of all LPG and greater than 95 percent of all CNG hydrocarbon exhaust constituents to be composed of C1 to C3 compounds. In addition, toxic emissions from the combustion of CNG and LPG were as low as 10 percent of those generated by combustion of gasoline. A comparison of ozone forming potential of the three fuels was made based on the Maximum Incremental Reactivity scale used by the California Air Resources Board. Post-catalyst results from stoichiometric operation indicated that LPG and CNG produced 63 percent and 88 percent less potential ozone than reformulated gasoline, respectively. On average over all equivalence ratios, CNG and LPG exhaust constituents were approximately 65 percent less reactive than those from reformulated gasoline. Finally, GC/MS analysis identified a number of nitrogen-containing hydrocarbon compounds in LPG and CNG exhaust.