<,正,>, The kinetics of polymerization of acrylonitrile （AN） initiated by quinquevalent vanadium (V,5+,)-thiourea （TU） redox system has been investigated in aqueous nitric acid in the temperature range from 30 to 50℃. The polymerization rate （R,p,） can be expressed as follows: In the copolymerization of acryionitrile with methyl acrylate （MA）, the reactivity ratios were found to be 1.0 and 1.1, respectively. The experimental observations suggest that the initiating species is probably a complex consisting of a central ion of Lewis acid-VO,2,+, and the ligands of Lewis bases-acrylonitrile, thiourea, and nitrate anions, while the initiating system in lower concentration, the polymerization of acrylonitrile does not occur if the thiourea is acidified prior to its reaction with quinquevalent vanadium. This indicates that the primary radicals （or the monomeric radicals in the present article） are produced by associated thiourea rather than isothlourea.