The reaction barriers for the addition of various gamma-substituted propyl radicals to alkenes were obtained via ab initio molecular orbital calculations. The reactions studied were the additions of 3X-propyl (X = H, F) to CH2 = CHF, 3X-propyl (X = H, CN) to CH2 = CHCN, and 3X-propyl (X = H, CN) to CH2 = CHNH2. The results indicate that the gamma substituent effect (i.e., penultimate unit effect) on the reaction barrier is strongly dependent on the conformations of the reacting radical and transition structure. The implications of these results for the penultimate unit effect in free-radical copolymerization are discussed.