The X-ray diffraction studies of the salt of 6,6-diethyl-5-oxo-5,6,7,8-tetrahydronaphthalene-2-carboxylic acid with (1S)-1-(4-methylphenyl)ethylamine were carried out at high pressure. The compound underwent the Norrish-Yang reaction in crystals brought about by UV–vis radiation. In order to understand the photoreactivity of the compound at high pressure, the structures of pure reactant crystals at 0.1MPa and 1.8GPa were compared in terms of the intramolecular geometrical parameters, the intermolecular interactions, the free space, the size of the reaction cavity and the cell parameters, which all changed with pressure. The course of the photochemical reaction was monitored at 1.0GPa and 1.8GPa. The changes in the unit cell parameters indicated that the reaction proceeded faster at 1.0GPa than at 1.8GPa, which was connected with the decrease of the volume of the reaction cavity and the increase of the intermolecular interactions at high pressure.