This paper describes experiments conducted to determine the ozone-forming potentials, specific reactivities, and reactivity adjustment factors for various heavy-duty engines operating on “industry average” (RF-A) gasoline, California Phase 2 gasoline, compressed natural gas (CNG), liquefied petroleum gas (LPG), and diesel fuel. Each engine/fuel combination was tested in triplicate using the EPA heavy-duty transient cold- and hot-start test protocol. Hydrocarbon speciation was conducted for all tests to allow for the determination of ozone-forming potentials, using California Air Resources Board maximum incremental reactivity factors as well as determination of the Clean Air Act “toxic” emissions.HYDROCARBON SPECIATION of exhaust emissions, in conjunction with the application of Maximum Incremental Reactivities (MIRs)(1)* for individual hydrocarbon species, is currently being used by the California Air Resources Board (CARB) as an integral part of the certification process for light-duty low emission vehicles. This process allows comparisons of ozone-forming potential to be made between various fuels and “industry average” gasoline. The result of these comparisons is a reactivity adjustment factor (usually less than 1.00 for alternative and reformulated fuels) which can be multiplied by the non-methane organic gas (NMOG) emission rate to obtain a reactivity-adjusted NMOG emission rate. Thus, CARB not only considers the hydrocarbon mass emission rates of light-duty vehicles, but also the potential for ozone formation.Although light-duty vehicles have received considerable attention regarding ozone-forming potential(1, 2, 3, 4, 5 and 6), only a limited amount of research has been reported on the hydrocarbon speciation of exhaust emissions from heavy-duty engines operating over the Environmental Protection Agency (EPA) certification transient emission test cycle.(7,8) In order to investigate the relative propensity of exhaust emissions from gasoline-, diesel-, compressed natural gas (CNG)-, and liquefied petroleum gas (LPG)-fueled heavy-duty engines to form ozone when tested over the EPA transient test cycle, South-west Research Institute (SwRI) sponsored an internal research program. During this program, emissions were measured from a variety of heavy-duty engines according to EPA protocol for transient emission testing of heavy-duty engines.