The free-radical polymerizations of methyl acrylate (MA) have been systematically studied under Co-60 gamma-ray irradiation using benzyl propane(dithioate) (a), benzyl phenylethane(dithioate) (b), benzyl benzenecarbodithioate (c), benzyl 4-cyanobenzenecarbodithioate (d) and benzyl 4-methoxybenzenecarbodithioate (e) as control agents. The results indicate that Z-group of dithioester plays an important role in the polymerization behavior. When Z-group is alkyl or alkyl derivative, in the case of dithioester a and b, the polymerizations reveal quite good living characteristics except for a similar inhibition period of about 40 min, which may be attributed to the lower propagation rate coefficients during the initialization period. However, for dithioesters c, d and e, aryl as Z-group, the polymerizations show very strong inhibitions in the processes. The strong inhibition may be associated with the formation of probable 3 or 4-arm structure compounds. It is worth noting that it is the first report for dithioester a to be used as control agent in 'controlled/living' free radical polymerization under gamma-ray irradiation. (c) 2005 Elsevier Ltd. All rights reserved.