A detailed mechanism for isoprene photooxidation is developed that includes the recent developments on each of isoprene's atmospherically important reactions: O3, OH, O(3P), and NO3 as well as the OH reactions of methacrolein and methyl vinyl ketone. The mechanism also attempts to account for the gaps in the product distributions for a few reactions. The CO yield from isoprene oxidation in the presence of NOx, more than about 50 ppt, is calculated as about 3.4. The mechanism is tested against chamber data that included a range of mixtures of the primary oxidants, isoprene concentrations, and hydrocarbon/NOx ratios. It performs well under conditions where the OH and O(3P) reactions dominate and predicts peak O3 concentrations within about 25% under all conditions. The mechanism underpredicts methyl vinyl ketone yields as well as O3 levels under conditions of low avaiable NOx. Sensitivity of the mechanism to assumptions about the NO3 and O3 reactions are examined, and possible sources of the discrepancies are discussed. A condensed mechanism is developed and compared to the explicit mechanism. ¿American Geophysical Union 1992