The cooperative effects were monitored in 2-benzyl-5-benzylidenecyclopentanone single crystals by the use of X-ray diffraction methods. The remarkable changes take place in the reciprocal lattice of the crystal after UV irradiation, indicating that the cooperative effects work and the crystal transformation proceeds. The mutual position of the substrate molecules and their internal geometry change with the reaction progress. This is explained in terms of the chemical pressure caused by the product molecules in the crystal. (C) 2002 Elsevier Science B.V. All rights reserved.