The photo-oxygenation of 2-(methoxymethylidene)adamantane (3) creates a zwitterionic peroxide which may be captured by acetaldehyde to give the corresponding pair of diastereoisomeric tricyclo[3.3.1.1^3,7]decane-2-spiro-6′ -(3′ -methyl-5′ -methoxyl′, 2′, 4′ -trioxanes) (4). Ease of capture depends strongly on solvent polarity and temperature. When these are low, yields of trioxine are high (∼ 80%). Conversely, 1,2-dioxetane formation is favoured at high temperature and solvent polarity. 2-(Phenoxymethylidene)adamantane (5), on photo-oxygenation, only gives the corresponding 1,2-dioxetene, even in the presence of acetaldehyde. From a Hammett, study of the-oxygenation of the enol ether 5 and p-methoxy, p-methyl, p-chloro and m-chloro derivatives, 9, 11, 13, and (15), a good linear relation was found between substituent constants and oxygenation rates which yielded reaction constants (ρ) of 2.59, −2.40, −1.09, and −0.90 in benzene, AcOET, CH₂Cl₂, and MeOH respectively. This data to the formation of a zwitterionic peroxide which enjoys stabilization from its won substituents and by competing solvation and further explains the predominance of dioxetane to the detriment of trioxane formation.