The photochemistry of ozone trapped in solid nitrogen and mixed matrices has been reinvestigated using irradiation at 266 nm and infrared spectroscopy between 10 and 25 K. In this temperature range the most probable reaction channel appears to be the formation of metastable excited nitrous oxide molecules that fragment mostly into an oxygen atom and nitrogen. Recombination of O+O-2 is negligible, suggesting that the atomic oxygen that is formed from the deactivation of excited N2O is bound to the nitrogen molecule, preventing the reformation of O-3 and leading to a fast first-order photodecomposition of ozone in highly diluted samples. The experimental value of the photokinetic constant suggests that the photodissociation cross section of ozone in matrices at 266 nm is weaker than that measured in the gas phase.