The thermal degradation of poly (methyl methacrylate) (PMMA) , in the presence of manganese chloride has been studied by sealed tube reactions and thermogravimetric analysis coupled to FT-IR spectroscopy. From sealed tube reactions it was found that the degree of mixing of the MnC12 and PMMA has an important effect on monomer formation. In systems where the two components are simply poured together, the amount of monomer is about half that observed for the thermolysis of PMMA alone; when the two components are thoroughly mixed by dissolution in solvent, the monomer yield falls to zero. The TGA-FT-IR experiment on solvent mixed material does show the presence of monomer. In sealed tubes, monomer may not escape and must repolymerize, while in the TGA experiment the monomer is swept out before reaction may occur. Monomer production also commences at temperatures lower than those for degradation of PMMA alone. It is apparent that manganese chloride catalyzes both the degradation of polymer to monomer and the reoligomerization of this monomer. The gases that are produced include CO, C O P , CH3Cl, HCl, and CH,. A mechanism is proposed to account for all of these products and a manganese ionomer is the final product of the reaction.