The structures of the main products resulting from photocyclodimerization of the title compound 2 and of other 3-methyl-substituted oxacyclohex-2-en-1-ones (¼ dihydropyranones) were determined by X-ray crystallography. In connection, the 13 C-NMR chemical shifts of the cyclobutane C-atoms of these dimers allow a clear differentiation between head-to-head and head-to-tail regioisomers, all structurally related to those of isophorone (1).Introduction. -Cyclohex-2-enones undergo photodimerization to mixtures of (regio-and stereoisomeric) tricyclo[6.4.0.0 2,7 ]dodecanediones. Whereas the assignment of the structural parameters for such tricyclic dimers of cyclic enones bearing no substituents on the olefinic C-atoms is unequivocal by analysis of the corresponding AA'XX' patterns [1 -3], the structure determination of the corresponding dimers of cyclic enones bearing a substituent on one of the olefinic C-atoms is not straightforward. Both enantioselective GC analysis [4] as well as CI-MS [5] can be helpful, but only if these dimers are stable under these conditions. In a recent publication [6], we have proposed for the first time to use