The geometrical parameters governing the potential for the photocyclization reaction occurring in crystals of 2,3,4,5,6-pentamethylbenzophenone, C(18)H(20)O, (I), 1,3-diphenylbutan-1-one, C(16)H(16)O, (II), and 2,4,6-triisopropyl-4'-methoxybenzophenone, C(23)H(30)O(2), (IV), have been evaluated. Compound (IV) undergoes photocyclization but (I) and (II) do not, despite the fact that their geometrical parameters appear equally favourable for reaction. The structure of the partially reacted crystal of the photoactive compound, i.e. 2,4,6-triisopropyl-4'-methoxybenzophenone-3,5-diisopropyl-7-(4-methoxyphenyl)-8,8-dimethylbicyclo[4.2.0]octa-1,3,5-trien-7-ol (9/1), 0.90C(23)H(30)O(2).0.10C(23)H(30)O(2), (III), was also determined, providing structural evidence for the reactivity of the compound. It has been found that the carbonyl group of the photoactive compound reacts with one of the two o-isopropyl groups. The study has shown that the intramolecular geometrical parameters are not the only factors influencing the reactivity of compounds in crystals.