Abstract: In the method of a photo-oxidative treatment of tissue containing collagen tissue, the tissue is pre-treated with an aqueous conditioning solution prior to an irradiation with light. In this pre-treatment the tissue permeable to the solution is loaded with a dye contained in the solution, in particular with methylene blue. This dye, activated by light, catalyzes photo-oxidative reactions. In accordance with the invention a substance is added to the conditioning solution, the refractive index of which is greater than that of water. As a result of this additive, the tissue containing collagen assumes a greater transparency for light during the pre-treatment.