Rate constants for the reactions of S(P-3) atom with H-2, D-2, CH4, C2H6, C3H8, n-C4H10, and i-C4H10 have been measured at temperatures above 830 K by a laser photolysis-shock tube method coupled with atomic resonance absorption spectrometry. The activation energies for these reactions were found to be almost identical with the endothermicities, indicating the lack of pronounced barriers along the reaction coordinates, in contrast to the analogous reactions of O(P-3) atoms. Group additivity of the rate constants for primary and secondary C-H bonds was found to be a good approximation for the reactions of S(P-3) with alkanes. Ab initio (MRSDCI) potential energy surface calculations also revealed that the barrier height along the collinear S(P-3) + H-2 reaction coordinate is very small, only 8.3 kJ mol(-1) above the SH + H asymptote.