We present direct, pressure-dependent measurements of OH yields from gas-phase ozone-alkene reactions, using the Harvard HOx instrument to obtain sensitive, accurate, and precise OH concentrations. As in previous studies from our laboratory, steady-state [OH> is measured by laser-induced fluorescence (LIF); here, the accurate LIF calibration allows us to present absolute OH yields for the first time. To calibrate our original LIF system, we measure yields from ozone plus tetramethylethylene (TME) as a function of pressure. The pressure dependence agrees with our previous results, so our yield measurements are now of comparable accuracy to those from indirect studies. Prompt, low-pressure yields agree well with 1 atm yields measured over longer timescales, confirming that much of the OH arises from decomposition of stabilized carbonyl oxides. We also explore the pressure dependence of yields from ethene, isobutene, and isoprene. Yields from isobutene and isoprene are pressure-dependent, consistent with the formation of substituted carbonyl oxides. We observe no pressure dependence of the OH yield from ozone+ethene, finding instead a constant yield of 14%, in line with most 1 atm studies but in contrast with the results of another pressure-dependent study. ¿ 2001 American Geophysical Union