The anionic heterogeneous polymerization of acrylonitrile in petroleum ether by n‐butyllithium was studied under various experimental conditions. The dependence of the molecular weight on the total concentrations of the monomer and initiator was strongly affected by the momentary concentration of monomer in the polymerization mixture and by the order of their addition to it. Two polymerization mechanisms were found to operate simultaneously. When monomer was added to the catalyst solution at low rates, nonterminated “living” polymerization was essentially dominant, as was evident from the linear dependence of DP on [monomer] and 1/[BuLi]. Under such conditions DP was independent of the rate of monomer addition. Chain transfer to monomer was the dominant mechanism at high rates of monomer addition, as was evident from the independence of the degree of polymerization of both monomer and initiator concentrations. At intermediate rates of monomer addition the extent of each of the two mechanisms was appreciable, and DP increased with decreasing the rate of monomer addition. A two‐stage propagation step in which the growing carbanion of the C−Li+ ion‐pair was solvated by monomer, was suggested to be responsible for the dependence of the polymerization mechanism on the momentary concentration of monomer in the polymerization mixture.