The copolymerization of acrylonitrile with methyl acrylate initiated by azobis(isobutyronitrile) in dimethyl sulfoxide at 60 degrees C has been investigated to high conversion using in situ C-13 NMR spectroscopy. The focus has been on the relative rates of individual monomer consumption as a function of time up to as high a conversion as experimentally feasible. It has been found that the relative rates of monomer consumption, i.e., the instantaneous copolymer composition, cannot be described by the low conversion values of the monomer reactivity ratios r(1) and r(2), These findings are discussed within the framework of possible effects of monomer diffusion rates and explicit penultimate unit effects in connection with monomer composition drift.