Density functional calculations are reported for the bond dissociation energy (BDE) of a number of dithioacetates, CH3C(S)S-R and selected dithiobenzoates, PhC(S)S-R, of relevance to reversible addition-fragmentation transfer (RAFT) controlled radical polymerization. In comparison with previously reported calculations [Gillies, M. B.; Matyjaszewski, K.; Norrby, P.-O.; Pintauer, T.; Poli, P.; Richard, R. Macromolecules 2003, 36, 8551-8559] at the same level on corresponding R-X systems (X = Cl, Br, I, N-3, S2CNMe2), the results reveal significant steric and polar effects on the BDE. Particularly bulky R groups (tBu, C(CH3)(2)COOMe) yield relatively weaker R-S(2)CZ (Z = Me, Ph) bonds, such that the radical transfer process to R'-S(2)CZ where R' is less sterically encumbering (e.g., CH(CH3)COOMe) is less favorable, when compared to the same transfer to R'-Cl (or R'-Br). As indicated by an analysis of DFT computed natural charges, electronegative substituents in the a. position of the R group (F, OMe, OAc, and also multiple substitution with Cl atoms) reinforce the ionic component of the R-X bond when X is a more electronegative group (i.e., Cl, Br) relative to S(2)CZ. Therefore, transfer of these radicals is also disfavored for R'-S(2)CZ relative to R'-Cl or R'-Br. These effects rationalize experimental observations and can be used as a guiding tool for the rational design of ATRP initiators and RAFT transfer agents.