Reactivities of volatile organic compounds (VOCs) from vehicle exhaust emissions, measured at the General Motors smog chamber facility, have been modeled using the SAPRC93 and its interim updated photochemical mechanism. The vehicle exhaust mixtures were generated by a single vehicle run over a portion of the Federal Test Procedure using three Auto/Oil Program reformulated test gasolines. For each run, up to 156 individual VOC species were identified. Initial nitrous acid (HONO) concentrations are needed to simulate reactivity measurement runs. (HONO is expected to be generated in a Tedlar bag used to collect the exhaust sample prior to its transfer to the smog chambers.) Measured and simulated relative incremental reactivities for the three exhaust mixtures are highly consistent. However, measured relative incremental reactivities are more sensitive to fuel effects than simulated ones. The maximum incremental reactivity (MIR)-based relative incremental reactivities, derived from individual species concentrations and MIR factors, are very close to simulated ones. A number of sensitivity simulation runs have been carried out to investigate the impact of HONO and other variables. Exhaust CO is shown to account for approximately 15% of the exhaust reactivity. The results show that the relative reactivities of actual vehicle exhaust emissions can be measured quantitatively by chamber runs in spite of the HONO effects.