An investigation of the causes of reddish discolouration in black-and-white photographs displayed in an exhibition. The mounted photographs had been enlarged on polyethylene paper, the substrate retouched with toned zinc white and mounted on a self-adhesive foil for protection x-ray analysis, microscopic examination and electron microscopy were applied. The discolouration may have been caused by an interaction between polyvinyl chloride in the mounting foil and the phenol proportion in the retouching paste. References.