High level ab initio calculations of the addition-fragmentation equilibrium constants at 333 K were performed for model RAFT reactions: R· + S=C(Z)SCH3 → R-SC·(Z)SCH3, for all combinations of Z = CH3 phenyl, or benzyl with R = CH3, benzyl, CH2COOCH3, or C(CH3)2CN. The results indicate that slow fragmentation of the polymeric RAFT-adduct radical is responsible for rate retardation in cumyl dithiobenzoate mediated polymerization of styrene and methyl acrylate. They also confirm that rate retardation should be relieved by changing the phenyl Z substituent to a benzyl group and that inhibition should be relieved by using a RAFT agent with a C(CH3)2CN leaving group. The equilibrium constants are extremely sensitive to the nature of the R and Z substituents, with a 13 orders of magnitude variation over the 12 reactions considered. The effects of these substituents on the reaction enthalpy are generally consistent with the trends expected on the basis of radical stabilization arguments. However, there are additional steric effects on the reaction entropy, and certain synergistic effects between the R and Z substituents on the stability of the RAFT-adduct radicals, which complicate the trends in the equilibrium constants.