The effects of monomer composition on the apparent chain transfer coefficient (< C-tr >) in reversible addition fragmentation transfer (RAFT) copolymerization were investigated. The studied RAFT systems included methyl methacrylate (MMA)/butyl acrylate (BA) mediated by 1-phenylethyl phenyldithioacetate (PEPDTA) (i.e. MMA/BA-PEPDTA), MMA/BA by 2-cyanoprop-2-yl dithiobenzoate (i.e. MMA/BA-CPDTB), and styrene (St)/BA by benzyl dithioisobutyrate (i.e. St/BA-BDTiB). The R groups of the RAFT agents were first converted to the corresponding copolymer oligomers having the same composition to facilitate the measurement of the main RAFT equilibrium transfer coefficients. It was found that there exist minimum values in the < C-tr > similar to f(1) curves in MMA/BA-CPDTB and St/BA-BDTiB at f(1) = 0.75 and 0.25, respectively. The apparent transfer coefficients of the copolymerization systems within some composition range were lower than their homopolymerization values. The lower < C-tr > values resulted in broader copolymer molecular weight distributions. The composition dependence Of < C-tr > was determined by the comonomer reactivity ratios and the Z group functionality of the RAFT agent. The experimental data could be well described by a simple equation derived from the terminal model: < C-tr > = < k(tr)>/< k(p)> = psi(1)r(1)f(1)C(tr,11) + r(2)f(2)C(tr,21)/r(1)f(1)(2) + r(2)f(2)(2) + 2f(1)f(2) + psi(2)r(1)f(1)C(tr,12) + r(2)f(2)C(tr,22)/r(1)f(1)(2) + r(2)f(2)(2) + 2f(1)f(2) (C) 2008 Elsevier Ltd. All rights reserved.