Products of the reaction of OH radicals with propene, trans-2-butene, and 1-butene have been investigated in a fast flow reactor, coupled with time-of-flight mass spectrometry, at pressures between 0.8 and 3.0 Toff. The product determination includes H atom abstraction channels as well as site-specific OH addition. The OH radicals are produced by the H + NO2 -> OH + NO reaction or by the F + H2O -> OH + HF reaction, the H or F atoms being produced in a microwave discharge. The gas mixture is ionized using single photon ionization (SPI at 10.54 eV), and products are detected using time-of-flight mass spectrometry (TOF-MS). The H atom abstraction channels are measured to be <2% for OH + propene, 8 +/- 3% for OH + 1-butene, and 3 +/- 1% for OH + trans-2-butene. Analysis of ion fragmentation patterns leads to 72 +/-16% OH addition to the propene terminal C atom and 71 +/- 16% OH addition to the 1-butene terminal C atom. The errors bars represent 95% confidence intervals and include estimated uncertainties in photoionization cross sections.