The polymerization of methyl methacrylate initiated by molybdenum carbonyl and carbon tetrachloride has been studied in dioxan, acetic anhydride, and benzonitrile solutions. Kinetic evidence suggests that monomer is involved in initiation, and that dioxan and acetic anhydride can also participate in the initiation process by a similar mechanism. A modified reaction scheme has been proposed to explain the unusual features of the benzonitrile system, in which two slow steps appear to be involved. Kinetic parameters have been evaluated, and experimental observations are in excellent agreement with theoretically determined rates. The implications of the values of the kinetic parameters have been discussed.