-The solid-state photolysis of oxiranes and aziridines produces highly colored ylide intermediates. The stability of these intermediates is dependent on a combination of electronic and steric factors as well as the solid-state constraints of the environment. _____ yX\Flash photolysis studies indicate that two different ylides can be produced in solution at room temperature. These intermediates appear to be identical to those formed consecutively in the solid state or lowtemperature glasses. By the use of certain gas-solid reactions, it is possible to control the lifetime of the intermediate so that it is destroyed immediately or continues to exist almost indefinitely. INTRODUCTIONThe room temperature photolyses of aryloxiranes have been shown by Griffin and co-workers to involve a two-bond cleavage producing aryl carbenes and carbonyl compounds (3,4). On the other hand, the thermal transformations of aryl aziridines were interpreted by Heine (5) and Huisgen (6) as one-bond cleavages producing azomethine ylide intermediates which could be trapped with suitable dipolarophiles.Our previous interest in low temperature and solid-state photochemical techniques (7-9) suggested the possibility of obtaining direct physical evidence for the above intermediates on photoreactions carried out at 77°K. The results of several of these studies already have appeared (10-13) and this report will describe additional recent work with special emphasis on the relation of the low-temperature studies to the unusual rooim-temperature solid-state photochemistry which is exhibited by many of the compounds. LOW TEMPERATURE PHOTOCHEMISTRY OF ARYL OXIRANESThe room temperature photochemistry of aryloxiranes has been studied extensively by Griffin and co-workers (3,4). The reactions involve a cycloelimination on photolysis in solution to give aryl carbenes and carbonyl compounds.