A series of experiments investigating the formation of OH radicals from O3-alkene reactions are described. Pairs of OH tracer compounds-aromatic and aliphatic hydrocarbons-were added to O3-alkene mixtures in small quantities. Under these conditions, a large fraction of the tracer is consumed, resulting in a much larger signal than in experiments using excess tracer. The average decay ratios of tracer pairs in 30 experiments were within 7% of those predicted by literature rate constants for tracer-OH reactions, strongly supporting the hypothesis that OH is formed directly from O3 reaction with alkenes. ¿ 1997 American Geophysical Union