Structural changes brought about by the Norrish-Yang reaction proceeding in two crystal forms of methyl 2-{[4-(2,4,6-triisopropylbenzoyl)benzoyl]amino}-3-phenylpropanoate were monitored by X-ray structure analysis. The S-enantiomer and the racemic mixture of the compound crystallize in the space groupsP2₁2₁2₁ with one molecule in the asymmetric unit and Pna2₁ with two symmetrically independent molecules, respectively. The changes in the cell volume for both crystal forms are of different kinds. They were attributed not only to the photochemical reaction but additionally to a loss of translational order in the racemic crystal. The structures determined for the partly reacted racemic crystal revealed that two symmetrically independent molecules react at different rates. This fact was explained quantitatively by means of differences in the free space and in the geometry of both molecules. It was also shown that only a single o-isopropyl group took part in the reaction.