This paper presents an approach to modeling the emission benefits of an inspection/maintenance (I/M) program. Emission benefits derived from an I/M program can be expected to be a function of many variables. The model considers the following variables: the level of the standard under which the vehicles were certified and their emission levels at the time of inspection, the vehicle deterioration rate characteristic of both the applied emission control technology and consumer maintenance practices, specifics of the particular I/M program (short test selected along with short test standards), and the quality of repairs made by the service industry. The major advantage of such a simulation model is that a wide variety of scenarios can be evaluated and the evaluations can examine time trends as inspection programs stabilize.The model outputs the I/M benefit, (percent and tons) for HC and CO, HC and CO failure rates, HC and CO omission error rates, and HC and CO comission error rates as a function of the number of inspection cycles to which the vehicle population has been subjected. Since the model operates with real emission input data on in-use vehicle populations, it makes no assumptions about the shape of emission distributions.The model was applied to the I/M program design currently in effect in Chicago. Average deterioration rate estimates were used. The model predicts 14 to 26 percent HC improvements for the first year of a mandatory enforced program with benefits increasing to 35 to 56 percent for a stabilized (10 year old) program. CO benefits are 17 to 22 percent and 50 to 66 percent respectively. The range of potential benefit in each case reflects the uncertainty associated with the quality of repair which can be expected from the service industry.