The branching ratios for the reactions of attachment of hydroxyl radical to propene and hydrogen-atom abstraction were measured at 298 K over the buffer gas pressure range 60-400 Torr (N-2) using a subatmospheric pressure turbulent flow react(); coupled with a chemical ionization quadrupole mass spectrometer. Isotopically enriched water (H2O)-O-18 was used to produce O-18-labeled hydroxyl radicals in reaction with fluorine atoms. The beta-hydroxypropyl radicals formed in the attachment reactions 1a and 1b, OH + C3H6 -> CH2(OH)C center dot HCH3 (eq 1a) and OH + C3H6 -> C center dot H2CH(OH)CH3 (eq 1b), were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions in O-2- and NO-containing environment. The O-18-labeling propagates to the final products, allowing determination of the branching ratio for the attachment channels of reaction 1. The measured branching ratio for attachment is beta(1b) = k(1b)/(k(1a) + k(1b)) = 0.51 +/- 0.03, independent of pressure over the 60-400 Torr pressure range. An upper limit on the hydrogen-abstraction channel, OH + C3H6 -> H2O + C3H5 (eq 1c), was determined by measuring the water yield in reactions of OH and OD radicals (produced via H(D) + NO2 -> OH(OD) + NO reactions) with C3H6 as k(1c)/(k(1a) + k(1b) + k(1c)) < 0.05 (at 298 K, 200 Torr N-2).