Résumé : The irradiation of an optically pure isomer of a spirooxaziridine, (2R,αS)-6(e)-tert-butyl-2-(α-methylbenzyl)-1,2-oxazaspiro[2.5.] octane, gave a single lactam, (5S,αS)-5-tert-butyl-1-(α-methylbenzyl)hexahydro-2-azepinone. The absolute configurations of these two compounds were established by chemical methods and X-ray crystal structure analysis. The comparison between the configurations of the reactant and product leads to the conclusion that the C-C bond cleaved is the bond in the anti position with respect to the lone pair of the oxaziridine nitrogen atom. This work provides clear evidence in support of the stereoelectronic theory that explains the regioselectivities observed in photochemical and thermal rearrangements of oxaziridines. The mechanism of the photochemical conversion of oximes (photo-Beckmann) can also be understood in terms of stereoelectronically controlled rearrangement of intermediate oxaziridines.