This report is a technical evaluation of the methodologies, assumptions and data used in the EPA's mobile source emissions model, MOBILE5a. Exhaust and nonexhaust emission rates and the effects of oxygenates are reviewed. The assumptions and extrapolations within the model are listed and critiqued. Sensitivity analyses are performed to assess the emissions effects of alternative methodologies in MOBILE5a. See also related documents Publ 4604 and 4605.