A small-ratio relative-rate technique to derive OH yields from O-3 reactions with alkenes is described. OH tracer compounds-aromatic and aliphatic hydrocarbons-are added to O-3-alkene mixtures in small quantities. Under these conditions, a large fraction of the tracer is consumed, resulting in a large signal that allows accurate determination of OH formation yields. Analytical approximations, numerical analysis, and sources of random and systematic uncertainties are discussed. This method is applied to measuring the OH formation yield from the O-3 reaction with propene and ethene, finding 0.35 +/- 0.07 and 0.18 +/- 0.06, respectively.