Abstract— The good transmission of near UV or visible light by human epidermis obtained through mild suction blisters makes it possible to perform quantitative absorption spectroscopy on various photosensitizing dyes such as hematoporphyrin and methylene blue incorporated into the epidermis. The variety of microenvironments found in the skin modifies the photophysical properties of chromo‐phores as shown by optical absorption band red shifts resulting from chromophore‐macromolecule interactions. They also modify the primary photochemical processes as exemplified by measuring the photobleaching quantum yield of the diphenylisobenzofuran, a singlet oxygen probe. The relevance of this study to photomedicine is discussed; in particular, it is shown that some psoralens used in human photochemotherapy can induce photosensitized reactions involving molecular oxygen.