The Hg3P1-photosensitized decomposition of benzaldehyde at low pressure and high radiation intensity forms predominantly a polymeric material. In the fraction of the reaction which forms the identifiable final products benzene and carbon monoxide, both a direct molecular rearrangement and a free radical split to phenyl and formyl radicals occur. The decomposition reactions of acrolein and crotonaldehyde involve the three primary processes:[Formula: see text]