Rate constants for the reactions of atomic oxygen (P-3) With selected alkanes (C-2-C-6 Straight chain alkanes, C-C6H12, neo-C5H12, and i-C4H10) have been determined directly by a laser photolysis-shock tube-atomic resonance absorption method at high temperatures (850-1250 K). For n-C6H14 and c-C6H12, rate constants have been also determined at lower temperatures (296-400 K) by a laser photolysis-vacuum ultraviolet laser-induced fluorescence method. Present results agree with the TST calculation by Cohen and Westberg [Int. J. Chem. Kinet. 1986, 18, 99] within experimental error limits except for O + c-C6H12 and O + neo-C5H12. The group additivity of the rate constants for different C-H bonds (primary, secondary, and tertiary) is examined and neo-C5H12 and c-C6H12 are found to be good representatives of primary and secondary C-H bonds, respectively, for C-4 and larger alkanes. A trial is made to investigate the J dependence of reactivity of atomic oxygen (P-3(J)) With c-C6H12 to examine the adiabatic correlation of reactants and products. No evidence for such J dependence can be observed due to the rapid relaxation among the spin-orbit components. Rate constants for the intratriplet relaxation processes are evaluated for He as a collision partner.