Radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) can be used to synthesize a wide range of polymers of controlled architecture and narrow molecular weight distribution. The polymerization use addition-fragmentation chain transfer agents (RAFT agents) that possess high transfer coefficients in free radical polymerization and confer living character on the polymerization. This paper explores the effect of the substituents R of dithiobenzoate RAFT agents [S=C(Ph)S-R] on the outcome of polymerizations of styrene, methyl methacrylate (MMA) and butyl (BA) or methyl acrylate (MA). In MMA polymerization at 60 °C, effectiveness depends strongly on R decreasing in the order where R is: -C(Alkyl)2CN ∼ -C(Me)2Ar > -C(Me)2C(=O)O(alkyl) > -C(Me)2C(=O)NH(alkyl) > -C(Me)2CH2C(Me)3 ≥ -C(Me)HPh > -C(Me)3 ∼ -C(Me)3 ∼ -CH2Ph. Of these, only the compounds with R = -C(Me)2Ph and -C(Me)2CN provided polymers with substantially narrowed polydispersities in batch polymerization and gave molecular weight control of the form expected for a living polymerization. These compounds have high transfer coefficients in MMA polymerization (transfer coefficient >20 for 0.003 M RAFT agent in bulk MMA). Reagents with R = -C(Me)2C(=O)O(alkyl), C(Me)2C(=O)NH(alkyl), -C(Me)2CH2C(Me)3, -C(Me)HPh, -C(Me)3, and -CH2Ph appear relatively ineffective in MMA polymerization because they have much lower transfer coefficients (ca. 1.7, 0.7, 0.4, 0.15, 0.03, and 0.03 respectively). In polymerizations of styrene or acrylates at 60 °C, transfer coefficients of RAFT agents are several orders of magnitude higher and all compounds in the above series yield narrow polydispersity polymers. The transfer coefficients of benzyl dithiobenzoate in styrene and methyl acrylate polymerization at 60 °C are ca. 50 (0.0093 M RAFT agent in bulk monomer) and 105 (with 0.003-0.006 M RAFT agent in 4.43 M monomer), respectively. The rate constant for addition to the thiocarbonyl group does not depend strongly on R. Differences in activity depend on the properties of R and the propagating chain as free radical leaving groups and on the partitioning of R• between adding to monomer (to reinitiate) and adding to polymeric RAFT agent. When the reaction of R• with the polymeric RAFT agent to reform the initial RAFT agent is significant, the transfer coefficient may reduce with concentration of RAFT agent and with conversion. In these circumstances, the activity of RAFT agents is best characterized by two transfer constants associated with the forward and reverse reactions. Steric factors, polar factors, and the stability of R• all appear to play an important role in determining the transfer coefficient and the effectiveness of RAFT agents.