19 photo substances were studied by thin‐layer chromatography (TLC) as fresh solutions, and during storage under various conditions. Chlorhexidine was excluded as it is sparingly soluble and only used in commercial products as a derivative. Trichlorocarbanilide was the only substance shown to be impure from the start. Only benzocaine, buclosamide, chlorhexidine acetate and chlorhexidine gluconate remained unchanged to the naked eye, and in TLC. The other substances were changed in colour and/or in their TLC‐pattern. An extraction method is described which makes it possible to use TLC for the examination of substances in petrolatum preparations. UV‐absorption spectra have also been examined for all the subst