The single-crystal-to-single-crystal photodimerization reaction of 2-benzyl-5-benzylidenecyclopentanone was studied in situ using X-ray diffraction. It was found that the degree of monomer conversion could be precisely measured from the change in X-ray diffraction intensities, leading to the determination of a rate constant. The rate constant was found to increase linearly as the temperature increased to similar to 200 K, yielding an activation of similar to 13 kJ mol(-1). Above this threshold temperature, the rate constant was found to decrease. Structure analyses of the monomer crystal at various temperatures revealed that below 200 K the intermolecular distance of two reactive olefin groups in adjacent molecules remained at a constant 4.15 Angstrom, yet this distance expanded above 200 K. The thermal parameters of the olefin carbon atoms increased linearly over the temperature range 132-324 K. These results suggest that the reaction is dominated by thermal vibration of the lattice below the threshold temperature while above it the reaction is likely to be dependent on the expanding intermolecular carbon-carbon distance. This is the first quantitative study of such a solid-state reaction.