The relative rates of conversion of monomer to polymer were found for the photoinitiated bulk polymerization of various ᵬ-substituted ethyl acrylates, x-CH2CH2OOC-CH˭CH2. These data were developed for systems in vacuum, with inert flush, with and without inhibitor present. Some plausible suggestions about mechanism are discussed. Data, however, do not permit firm conclusions regarding the mechanism.