The copolymerization of styrene and acrylonitrile has been investigated in bulk and in the solvents toluene, butanone, and N,N’-dimethylformamide. The comparison of the copolymers prepared in the bulk system with copolymers obtained from copolymerizations in the various solvents reveals a relatively small but noticeable solvent effect. All the systems display a significant penultimate unit effect. From this work it is concluded that the copolymerization of styrene and acrylonitrile in the employed solvents can be explained by the qualitative description of the bootstrap mod i 1. The comparison of the bulk with the toluene system seems to indicate that the quantification of the bootstrap model is not realized by the assumption that the distribution coefficient K is constant. A modification of the earlier proposed method for quantification of the bootstrap effect is suggested. The distribution coefficient is linearly dependent on copolymer composition. The new method provides a good fit with experimental results, whereas the used parameters have a distinct physical meaning.