The effects of the R- and Z-substituents on radical stability in the reversible addition fragmentation chain transfer (RAFT) polymerization process have been studied via high level ab initio molecular orbital calculations. Radical stabilization energies (RSEs) of the RAYT-adduct radicals CH3-SC.ZSR and corresponding leaving group radicals R-. have been calculated for various combinations of Z = H, Cl, C equivalent to CH, CH=CH2, CN, CF3, NH2, CH3, Ph, Bz, naphthyl, OCH3, OCH2CH3, OCH(CH3)2, and OC(CH3)(3) and R = CH2CN, C(CH3)(2)CN, Bz, CH(Ph)CH3, C(Ph)(CH3)2, CH2COOCH3, CH(COOCH3)CH3, CH2OCOCH3, and CH2CH3. The results were used in combination with the corresponding values of the enthalpies of the fragmentation reactions, CH3SC.(Z)SR --> CH3SC.(Z)=S + R-. and CH3SR(Z)SR --> CH3 + S=C(Z)SR, to examine the effects of the substituents on the stability of both the RAFT-adduct radicals and the corresponding thiocarbonyl compounds. The RAFT-adduct radicals are stabilized by electron donation from the two sulfur substituents, and this stability can be further enhanced by unsaturated pi-accepting substituents (such as CN, phenyl, and naphthyl). In contrast, lone pair donor Z-substituents (such as Cl, NH2, and OCH3 have a much smaller effect on radical stability. The R-group, which can modify the donation ability of the SR-group, has a minimal effect on the stability of the RAYT-adduct as it is buffered by the second sulfur substituent. However, these orbital interactions do affect the strength of the breaking S-R bond, and this provides an important contribution to the trends in the fragmentation enthalpies. Steric effects on radical stability are also important, with bulky R- and Z-groups inducing conformational changes that interfere with these orbital interactions, sometimes with unexpected consequences. The substituent effects on the RAFT agents are qualitatively different; the agents are strongly stabilized by the lone pair donor Z-substituents and strongly destabilized by electron withdrawing groups (such as CN and CF3) in the R- and Z-positions. Moreover, steric effects are generally more significant, with bulky R- and Z-groups destabilizing the RAFT agent more than the corresponding RAFT-adduct radicals. As part of this work, the accuracy of the low-cost RMP2/6-311+G(3df,2p) method for studying addition-fragmentation processes in RAFT polymerization was evaluated.