The 310 nm irradiation' of cyclopropanecarbaldehyde (AH, 1M) in inert solvents2 yields as major products butanal (l_), dicyclopropylethanedione (DK), 1,2-dicyclopropylhydroxyethanone (2_) and 1,2,3-tricyclopropyl-2-hydroxy-1,3-propanedione (3_). E.g. irradiation of 0.7M AH in neopentane at -IO0 yields these products in a ratio of 4:3:5:2 respectively. Similarly, 1,2,3-tricyclobutyl-2-hydroxy-1,3-propanedione is eventually a major photoproduct on > 300 nm irradiation of cyclobutanecarbaldehyde in inert solvents at -60' 2. The (une?pected) formation of 2 has to proceed via a multi-step mechanism, since a collision between three cyclopropanecarbaldehyde molecules, one of which is electronically excited, is statistically unlikely. In order to obtain mechanistic information, a detailed study on the formation of 2 from AH was made. The results are compiled in the Table. In all irradiations 2 and 3 are major products. The product formation of exp. I, monitored with g.l.c., revealed i) that 1, 1 and DK are primary products, ii) that 2 is formed after an induction period, and iii) that the formation of 3 occurs at the expense of DK. Thus DK is involved in the formation of 2. Kelder and Cerfontain showed that photoexcited DK easily abstracts the methine-H of 2-propanol3. For the present system we suggest a similar step, viz. abstraction of the loosely bonded acyl-H of AH by photoexcited DK (i), fcllowed by combination of the thus formed radical pair (ii). The result of exp. II supports this proposal. DK*+AH -DKH' 'A (i) DKH' 'A -2 (ii) AH* + DK b AH + DK* (iii)