Mercury- (3P1) sensitized decomposition of 3,4-dihydro-2H-pyran in the vapor phase gave rise to ethylene, acrolein, and cyclobutane carboxaldehyde as the principal products. The molecular split which gave ethylene and acrolein had a maximum quantum yield of 0.14. The isomerization reaction had a quantum yield of 0.02 at ca. 50 Torr. In preparative runs at 1-atm pressure and 84.5°, there was a 20% yield of cyclobutanecarboxaldehyde. Photolysis of 2,5-dimethyl-3,4-dihydro-2H-pyran-2-carboxaldehyde in solution in cyclohexane with 300-nm radiation gave 2,5-dimethyl-2H-pyran, possibly by a free-radical process, and 1,2-dimethylcyclobutane- 1,2-dicarboxaldehyde by a ring-contraction reaction. It is suggested that the photoisomerization of 3,4-dihydro-2H-pyrans to cyclobutanecarboxaldehydes is a general process that is analogous to the photochemical ring contraction that is known in 2,3-dihydrofurans and furans.© 1970, American Chemical Society. All rights reserved.