The OH radical-initiated photooxidation of isoprene has been investigated experimentally by a 6-m3 photochemical reaction chamber equipped with a long path length Fourier transform infrared spectrometer. In the presence of NOx, the major primary reaction products were methyl vinyl ketone, methacrolein, and formaldehyde. Their yields were in quantitative agreement with previous measurements. In the absence of NOx, the reaction mechanism was found to be quite different from that in the presence of NOx, and major reaction products observed in the infrared spectra were attributed to organic hydroperoxides. Based on the model experiments, the ultimate yield of CO was evaluated to be 60% on the carbon number basis in the presence of NOx and 23% in the absence of NOx. The CO yield in the real atmosphere was evaluated as 30% on the carbon number basis, and global annual CO production from isoprene was estimated to be 105 Tg C yr-1. Together with a previous estimate of the CO production from terpenes, global CO production from natural hydrocarbons was evaluated to be 200¿60 Tg C yr-1. ¿ American Geophysical Union 1994