The invention provides compounds for use in photochemotherapy or diagnosis, said compounds being branched alkyl esters or substituted alkyl esters of 5-aminolevulinic acid, or derivatives or pharmaceutically acceptable salts thereof. In particular, the invention provides compounds of formula (I): R.sup.2.sub.2N--CH.sub.2COCH.sub.2CH.sub.2CO--OR.sup.1 (wherein R.sup.1 represents an optionally substituted branched alkyl (e.g. C.sub.5-30 alkyl) group, or a substituted alkyl group; R.sup.2, each of which may be the same or different, represents a hydrogen atom or an optionally substituted alkyl group, preferably a group R.sup.1; wherein said substituents are selected from hydroxy, alkoxy, acyloxy, alkoxycarbonyloxy, amino, aryl, nitro, oxo, fluoro, --SR.sup.3, --NR.sup.3.sub.2 and PR.sup.3.sub.2 groups, and each alkyl group is optionally interrupted by one or more O--, --NR.sup.3--, --S-- or --PR.sup.3-- groups; and R.sup.3 represents a hydrogen atom or a C.sub.1-6 alkyl group) and salts thereof for use in diagnosis and photochemotherapy of disorders of abnormalities of external or internal surfaces of the body, and products and kits for performing the invention.