solutions and by the direct observation of reactive intermediates in matrices at cryogcnic temperatures. Irradiation of 19 in methanol gave the acetal 22. The isolation and spectroscopic detection of intermediate photoproducts revealed that 22 was formed through the diazo ketone 24 and then the benzocyclobutene 26. In benzene, the bi(norcaradiene) 29 was formed via the diazo ketonorcaradiene 30. In