OH radical formation from the ozone reaction with isoprene, α-pinene, and methyl vinyl ketone were measured using the recently developed small-ratio relative-rate technique. This method uses small amounts of fast-reacting aromatics and aliphatic ethers to trace OH formation. Measured OH yields were 0.25¿0.06, 0.70¿0.17, and 0.16¿0.05 for isoprene, α-pinene, and methyl vinyl ketone, respectively. The levels of biogenic alkenes necessary to contribute a non-negligible fraction of new OH, HO2, and RO2 radicals are briefly discussed. ¿ 1998 American Geophysical Union