Through the study of the copolymerization of AN and MMA in bulk and in solution of N，N-dimethylformamide，dioxane，benzene，cydohexane，methanol and water，the fol-lowing results have been obtained：1．In solution precipitation copolymerization，it has been proved that solvents do influence the monomer reactivity ratios for certain monomer pairs．2．The copolymer composition curves and the monomer reactivity ratios of such co-polymerization system in different solvents were obtained．It has been proved that the result of the copolymerization of AN and MMA in water obtained by Sadao Yuguchi[6] was not correct．3．According to the above experimental results．we incline to believe that the change of the copolymer composition in precipitation copolymerization seems to be due to the difference between the effective composition of the reacting monomers in the reaction site and the average composition of the monomer in the system．The effective composition of the reacting monomers then would be reasonably assumed to relate both with the dif-ference between the polarities of the two monomers and witll the difference between the polarities of the liquid medium and the solid copolymer．4．In precipitation copolymerization，the concentration of monomer also influences the copolymer composition in certain cases．