The rosebengal-sensitized photooxygenation of 2-methoxynorborn-2-ene(1)in acetaldehyde gave cis-1-carboxaldehyde-3-carbomethoxycyclopentane (31%) and the cis and trans-Me derivatives of the cis-fuscd exo-1,2,4-trioxanes arising by addition of a molecule of oxygen and acetaldehyde to 1 at C3 and C2 respectively (13%) Similar photooxygenation of 2-(methoxymethylidene)adamantane in the presence of acetaldehyde, propionaldehyde and pivalaldehyde gave adamantanone (31–42%), and the cis and trans tricylo[3.3.1.1^3,7]decane-2-spiro-6'-[3-alkyl-5-methoxy-1,2,4-trioxanes] in yields of 32–53% Trioxane formation under similar conditions was experienced for 1,1-di-t-butyl-2-methoxyethene and 2-(methylmercaptomethylidene) adamantane. The results are discussed in terms of an intermediate zwitterionic peroxide which can either close directly to a 1,2-dioxetane or, if aldehyde is present, condense across the CO function giving the 1,2,4-trioxane.