AbstractThe photoinitiation efficiency of the free‐radical polymerization of methyl methacrylate and styrene by several carbonly compounds has been determined. The compounds considered were α‐substituted ketones and α‐dicarbonyl compounds. For the ketones, the initiation efficiency employing methyl methacrylate depends on the α substitution; the values obtained change from less than 10−3(acetone) to 0.65 (3‐hydroxy‐3‐methyl‐2‐butanone). All ketones are more efficient towards methyl methacrylate than styrene. This result can be explained in terms of triplet quenching by the last monomer. The results obtained employing α‐dicarbonyl compounds do not conform to a simple pattern. In particular, benzil shows a considerably larger efficiency towards styrene than for methyl methacrylate. Since benzil is efficiently quenched by styrene, the initiation must involve the interaction of an excited benzil m