The OH radical is the key oxidizing agent in the troposphere, and ozone-alkene reactions appear to be a significant and sometimes dominant source of new HOx radicals in urban and rural air. In this work, we report the first study of the pressure dependence of the OH radical yield for the ozonolysis of ethene, propene, I-butene, trans-2-butene, and 2,3-dimethyl-2-butene over the range 20-760 Torr and of trans-3-hexene and cyclopentene over the range 200-760 Torr. Low-pressure experiments were performed in a long-path evacuable FTIR cell or a steady-state flow-tube reactor in series with a gas chromatograph/flame ionization detector and FTIR cell. We have also investigated the effect of adding SF6 at atmospheric pressure for ethene, I-butene, and trans-2-butene, in a collapsible Teflon chamber. OH formation increased almost 3-fold for ethene at low pressures, from 0.22 +/- 0.06 at 760 Torr to 0.61 +/- 0.18 at 20 Torr, and increased somewhat for propene from 0.33 +/- 0.07 at 760 Torr to 0.46 +/- 0.11 at 20 Torr. A pressure dependence of the OH formation yield was not observed for I-butene, trans-2-butene, 2,3-dimethyl-2-butene, trans-3-hexene, or cyclopentene over the ranges studied. Density functional theory calculations at the B3LYP/6-31G(d,p) level are presented to aid in understanding the trends observed. They lead to the proposal that the formation of a hydroperoxide via a diradical pathway can compete with the formation of the carbonyl oxide for the ethene primary ozonide.