Synopsis A rapid method has been developed in order to compare the photostability of several sunscreen agents incorporated in the same type of emulsion. Thin films of the different preparations were spread on quartz plates and irradiated with a solar simulator. Differences in energy distribution according to wavelengths observed with solar simulator and sun radiation were taken into account. A kinetic study of absorption properties was carried out under solar simulator irradiation. A simple calculation gave the results which would be obtained under sunlight. This technique differentiated the reversible transformations of photoisomerizable compounds from an irreversible disappearance which resulted in a significant loss of protective power. Applied to benzylidene camphor derivatives, this technique showed the excellent photostability of these sunscreen agents.