Abstract The basic structure of benzaldehyde photopolymer (BPOL) consists of phenyl groups attached to a carbon-carbon and carbon-oxygen chain. The average BPOL molecule contains 12 monomer units. Relative rates for the BPOL photosensitized dimerization of 1,3-cyclohexadiene and the isomerizations of the 1,3-pentadienes, cis-1,2-dichloroethylene and cis-2-butene are reported. The BPOL photosensitized isomerizations of the 1,3-pentadienes in the visible ( λ > 395 nm) proceed by a simple mechanism. Stern-Volmer plots are linear and the photostationary trans/cis ratio is 2.0, independent of 1,3-pentadiene pressure over the range 2–180 Torr. The energy of the BPOL triplet state is less than but near 53 kcal mol −1. It excites the s- cis 1,3-pentadiene rotamers exclusively. The decay ratio of the s- cis triplets is 3.75.