<正> The dependence of the molecular weights on the concentration of reactants in the polymerization of acrylonitrile initiated by vanadium （V）-thiourea redox system has been investigated. It was found that the molecular weights of the polymer change nonlinearly with increasing concentrations of nitric acid and thiourea. Probably, the composition of the complexes exert a great influence on the chain initiation and termination. The reaction of "complextermination" gives rise to the decrease of the molecular weights markedly while the concentrations of thiourea and vanadium （V）in the range from one to three molar ratios.