Co-oligomers and copolymers of styrene and methyl methacrylate have been prepared by free radical polymerisation in the presence of the chain transfer agent ethyl-alpha-(t-butanethiomethyl) acrylate. Chain transfer constants are not significantly influenced by the co-monomer composition and lie in the range 0.47-0.90. Bulk copolymer composition is independent of the amount of chain transfer agent in the higher MW range (> 20 000). In contrast, the styrene content of lower MW oligomers extracted by fractional precipitation decreases with decreasing MW below about 1000. The efficiency of the addition-fragmentation mechanism in producing specifically end-functionalised co-oligomers (MW < 5000) was investigated by H-1 n.m.r. spectroscopy. Spectral peaks are mostly consistent with the expected end groups for all co-monomer feed systems. Quantitative consistency of end group concentrations is also good for co-oligomers with MW > 1500. At lower MWs, there is an increasing deficiency in olefinic end groups and a high ester group concentration relative to that expected from the MW.