The present invention provides photosensitizing agents obtained by reducing a single double bond in the porphyrin macrocycle of a sulphonated meso-tetraphenylporphyrin, preferably a disulphonated meso-tetraphenylporphyrin such as TPPS2a. The resulting sulphonated meso-tetraphenyl chlorins include compounds of formula I: (wherein X is —SO3H; n, p, q and r are each independently 0 or 1; and the sum of n, p, q and r is an integer from 1 to 4, preferably at least 2, e.g. 2 or 4) isomers and isomer mixtures thereof. The compounds of the invention and their pharmaceutically acceptable salts find particular use as photosensitizing agents in the photochemical internalization of molecules and in photodynamic therapy.