The rate constants for the addition reactions of the para-substituted phenylthiyl radicals (ZC₆H₄Ṡ) to disubstituted olefins (CH₂CXY) have been determined by the flash photolysis method. The substituents X and Y in the olefins were chosen in order to obtain kinetic evidence for a captodative effect on the free-radical addition reactions in which the captodative radicals (PhSCH₂ĊXY) were formed. When X was an electron-withdrawing (captive) group such as CN, electron-donating (dative) substituents (Y) accelerated the reactions of the olefins with the arylthiyl radicals. The order of acceleration is as follows: NR₂ > OR > Cl > OC(O)CH₃ > CH₃ > H. This result was interpreted in terms of a lowering of the activation energy caused by captodative stabilization of the transition state. For substituted propenes [CH₂C(Y)CH₃], the rate constants for addition increase with the electron-withdrawing power of the substituents Y. Localization energies for captodative-substituted olefins, from MO calculations, are well correlated with the kinetic data obtained in this study.