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The solid state [ 2 + 2] cycloaddition reaction of coumarin-3- carboxylic acid, 1a, has been investigated using both single-crystal and powder X-ray diffraction techniques which established the structure of the dimerized product, 1b. The transformation was achieved by irradiation with a 150 W mercury lamp for 3 d. Monomer pairs in the starting material, 1a, arrange such that the potentially reactive C] C double bonds are related by an inversion centre and separated by 3.632( 3) A. In the structure of the irradiation product, 1b, this distance reduces to 1.617( 7) A, with the formation of the planar cyclobutane ring, while maintaining the monoclinic P2(1)/ n symmetry of the reactant lattice during the reaction. Attempts to photodimerize two other compounds, 5-bromouracil 2a and maleic acid 3a, whose monomer molecules, within the crystal, were appropriately orientated and with C] C double bond separations of less than 4.2 angstrom, were also undertaken. However, using the same irradiation conditions as for 1a no observable changes occurred for either 2a or 3a. These observations can be rationalised by the presence of a much more extensive hydrogen bonding network than is present in 1a.