A systematic investigation has been conducted of the effect of various phenols (mono- and bicyclic) on the potassium mono-persulfate-initiated polymerization of methyl methacrylate catalyzed by Na2S2O2in aqueous medium at 45°C. It is found that all the phenols, irrespective of their low or high transfer constants, behave as weak retarders within a certain concentration range (0.0005–0.0025 M), above which they behave both as inhibitors and retarders. The rate of this process is dependent upon the substituents in the phenols and often on their positions. For comparison, the reaction has also been studied in the absence of phenols, and the effect of monomer, initiator, and temperature on the rate has been noted. The mechanism of inhibition and retardation has been proposed and various rate parameters have been evaluated.