The effects of C6H5PCl2, POCl3, and PCl3 on the free radical polymerisation of acrylonitrile initiated by a,a′-azobisisobutyronitrile were kinetically investigated in benzene and trichloroethylene solutions. With PCl3 rates of polymerisation were unaffected but the degrees of polymerisation were found to decrease with increasing PCl3. With C6H5PCl2 and POCl3 however both rates and degrees of polymerisation were adversely affected. Thus PCl3 acts as a chain transfer agent while the other two phosphorous compounds function as degradative chain transfer agents. © 1980 Springer-Verlag.