The site-specific branching fractions for the reaction of O(P-3) atom with C3H8 [--> OH + n-C3H7 (1a); --> OH + i-C3H7 (1b)] and the subsequent OH-radical reaction with C3H8 [--> OH + n-C3H7 (2a); --> H2O + i-C3H7 (2b)] have been studied experimentally at temperatures 593, 944, and 1130 K. Since it was difficult to separate the O-atom reaction from the rapid subsequent OH-radical reaction, the sum of the branching fractions for (1b) and (2b) was determined by measuring the yield of i-C3H7 radicals. Two methods are presented and have been tested for the discrimination of alkyl isomers. At low temperature (593 K), i-C3H7 radical was directly and selectively detected with photoionization mass spectrometry by utilizing the difference of ionization potentials of n-C3H7 and i-C3H7 radicals. At higher temperatures (944 and 1130 K), the yield of i-C3H7 radical was determined by using a laser photolysis-shock tube apparatus from the yield of H atoms, which are produced from the thermal decomposition of i-C3H7 radicals. It was confirmed experimentally that the i-C3H7 radical exclusively decomposes to H + C3H6 while the n-C3H7 radical mainly decomposes to CH3 + C2H4 and the minor decomposition pathway (--> H + C3H6) contributes little (<5%) in the present experimental conditions. By subtracting the reported branching fraction for (2b) [Droege, A. T.; Tully, F. P. J. Phys. Chern. 1986, 90, 1949], the branching ratio for the O(P-3) atom reaction was evaluated to be k(1a)/k(1b) = 2.5 exp(-8.9 kJ mol(-1)/RT) [593-1130 K], which well agrees with the transition-state theory calculation by Cohen and Westberg [Int. J. Chem. Kinet. 1986, 18, 99].