The invention relates to new derivatives of 5-aminolevulinic acid (5-ALA) and their use as photosensitizing agents. In particular, it relates to compounds of general formula I and their pharmaceutically acceptable salts, to methods for preparing such compounds and their medical and cosmetic use, for example in methods of photodynamic therapy and diagnosis : wherein R1 represents a hydrogen atom or an optionally substituted alkyl or cycloalkyl group; R2, each of which may be the same or different, represents a hydrogen atom or an optionally substituted alkyl group; and X is a linking group.