Six chemical reaction mechanisms for photochemical smog were used to study the effect of input parameters on volatile organic compound (VOC) control requirements needed to reduce ozone. The parameters studied were initial VOC composition, dilution rate, post 8-A.M. emissions, base case (present day) O3 levels, entrainment from aloft of VOC and ozone, initial HONO and initial VOC/NOx ratio. The Empirical Kinetic Modeling Approach (EKMA) was used to generate ozone isopleths for each chemical mechanism. The VOC control needed to reduce the maximum ozone concentration from some present day value to 0.12 ppm was calculated using the six mechanisms. The initial VOC/NOx ratio was found to have the largest effect of all the parameters studied on VOC control requirements. Ozone entrainment from aloft and the composition of the initial VOC mixture also had a large effect on predicted control requirements. To reduce the degree of uncertainty in control predictions, it is necessary to establish as accurately as possible the composition of urban air. Also, because of the substantial effect the choice of chemical mechanism has on predicted control requirements, it is important that efforts continue to be directed toward evaluating candidate mechanisms with respect to their ability to simulate atmospheric smog chemistry.