Source apportionment analysis for exposure to air toxics from conventional and oxygenated fuel was performed for different microenvironments. Personal toxic exposure data were taken from previous studies conducted in areas where MTBE oxygenated fuels were used. Refueling, commuting, and occupational microenvironments were all examined. The emission source, either tailpipe or evaporative, was estimated using the ratio of MTBE/benzene as an emission finger print. ASPEN simulations were completed to estimate the MTBE to benzene ratio for evaporative emissions from vapor above the fuel using vapor-liquid equilibrium models. Expected MTBE to benzene ratios in the tailpipe exhaust were obtained from previous studies. Refueling exposure was found to be dominated by evaporative emissions, specifically flash from the fuel tank for stations with Stage I controls, and evaporation of whole fuel for stations with Stage II controls. The occupational MTBE/benzene exposure ratio for mechanics was approximately the same as for refueling, but the absolute values were significantly higher. Exposure to commuters was dominated by exhaust emission. The levels of air toxics inside the vehicle cabin were the same as outside concentrations. These results can be used in conjunction with the EPA complex model for evaporative and tailpipe emissions, and a microenvironment behavior pattern, to establish an estimated health risk for a fuel based on its composition.