This paper describes the results of measurements of the branching ratio (k2b/(k2a+k2b)) for formation of organic nitrates via peroxy radical reaction with NO, following the reaction of the OH radical with isoprene (2-methyl-1,3-butadiene). The experiments were conducted in a 5 m3 all-Teflon photochemical reaction chamber, via the photolysis of isopropyl nitrite in the presence of isoprene and NO. The organic nitrate yield was determined from the measurement of the sum of all organic nitrate isomers, using an organic nitrate selective detector, as a function of isoprene consumed. Organic nitrates were sampled directly from the reaction chamber into a capillary chromatographic column, followed by separation and quantitative pyrolytic conversion to NO2, which was detected through luminol chemiluminescence. In this manner, seven isomeric organic nitrates were observed, with a total yield of 4.4%. The structural features of the precursor peroxy radicals that influence the magnitude of the yield is discussed. Emission inventories for isoprene and NO lead to the conclusion that as much as 7% of NO emitted in the eastern United States in the summer months is lost from the atmosphere through the isoprene nitrate channel. ¿ 1998 American Geophysical Union