The mechanism and kinetics of retardation of peroxydisulfate/thiourea-initiated polymerization of acrylonitrile (AN) by phenol have been studied in the temperature range of 40°–50°C. The effects of the concentration of retarder (phenol), monomer (M), peroxydisulfate (S2O82-), and thiourea (TU) on polymerization have been studied. The most remarkable observation in the present investigation is the positive intercept obtained from the plot of [M]/Rpvs. 1/[M]. A reaction mechanism has been suggested and a suitable rate law has been proposed on the basis of experimental findings.