In the reactions of H atoms, generated by the Hg (3 P 1)-sensitized decomposition of H2, with C2H5SC2H5 the primary reaction has been established to be with k 1=(4.7±0.9)× 1013 exp [-(3797±152)/RT] cm3 mol-1 sl relative to the rate constant of the H+ C2H4→ C2H5 reaction. The mechanism postulated explains adequately all the experimental observations. The entropy of activation for the primary reaction,-22.1 eu is close to the value obtained for ...