In attempts to homopolymerize benzaldehyde with various ionic and coordination catalysts, or by γ‐radiation, only the ionic catalyst BF3 etherate formed a polymeric product. This product was identified as polybenzyl with a degree of polymerization (DP) of 133. Mixtures of benzaldehyde and styrene did not copolymerize under γ‐irradiation, but yielded only polystyrene. With a BF3 etherate catalyst, however, mixtures of styrene and benzaldehyde were found to have copolymerized through the carbonyl group of the latter, and a 50 : 50 styrene–benzaldehyde monomer mixture yielded an 80:20 styrene–benzaldehyde copolymer with a DP of 139.