Bond dissociation enthalpies can exhibit dramatic variations resulting from substituent effects. These variations result from changes in electronic structure that accompany bond dissociation. We have studied bond dissociation enthalpies (BDEs) at the W1BD level of theory for a series of RX-H compounds where X = CH2, NH, O, PH, and S. The substituents, R, included H, H3C, H2N, HO, F, H2P, HS, and Cl. The experimentally available BDEs were reproduced in a very satisfactory fashion. Most of the substituent effects could be related to a conjugative interaction in the two-center three-RXH = RX. + H Delta H X = CH2, NH, O, PH, S R = H, H3C, H2N, HO, F, H2P, HS, Cl, CN, O2N, F-3 Celectron systems in the radicals formed by H abstraction. Comparisons of isoelectronic species demonstrate the important roles that variations in electronegativity and hybridization can play in determining BDEs. The OH and SH BDEs were linearly related, and the same was found with N-H and P-H BDEs. The relative effects of H2P and HS as substituents, in contrast to H2N and HO, could be related to the need to promote a 3s electron to 3p before H2P can participate in pi-bonding. We must also include electronegativity differences to account for the observed transfer of a sulfur lone pair to an oxygen but the absence of the reverse process.