Small amounts of residual monomer in poly(methyl methacrylate) (PMMA) have been estimated accurately by cross-polarization/magic-angle-spinning C-13 NMR spectroscopy. It has been verified that residual monomer decreases when PMMA is gamma-irradiated. In contrast to previous suggestions, a case is made that removal of monomer is due to slow postirradiation reaction with polymer free radicals. An additional unexpected finding is that radiolysis can result in the formation of additional monomer which can be detected in samples frozen immediately after irradiation.