The photochemistry of representative six and five-membered alpha,beta-unsaturated lactones [alpha-pyrone and some of its derivatives, including coumarin and 3-acetamidocoumarin, 2(SH)-furanone] isolated in cryogenic inert matrices has been investigated by infrared spectroscopy and quantum chemical calculations. In these types of molecules, two main competitive photochemical reaction pathways could be identified: ring opening, leading to formation of the isomeric aldehyde-ketenes, and ring contraction to the corresponding Dewar isomers. For alpha-pyrone and 2(5H)-furanone, the ring-opening process dominates over the ring-contraction reaction, the same occurring for derivatives of these compounds bearing a voluminous substituent at position 3. In 2(5H)-furanone, the ring-opening reaction requires the simultaneous occurrence of a [1,2]-hydrogen atom migration. Nevertheless, it was found to be an easy process upon excitation at lambda > 235 nm. The ring-opening reaction was also found to occur much easily in alpha-pyrone than in coumarin, and factors explaining this observation were discussed. In turn, the Dewar forms of the studied compounds resulting from the ring-contraction photoreaction were found to undergo subsequent photo-elimination of CO2, with formation of the corresponding cycloalkenes. In the matrices, CO2 and the simultaneously formed cycloalkenes were found to exist as associated forms, in which the CO2 molecule is preferentially placed over the cycloalkene ring in a stacked-type geometry. For coumarin, a third photoreaction channel was observed, leading to the formation of benzofurane and CO. This additional reaction channel corresponds to the photoreaction previously observed for the compound in the gaseous phase. Copyright (c) 2008 John Wiley & Sons, Ltd.