Partial syntheses of mixed functional phorbol- (12,13) -diesters with acetic acid and long-chain fatty acids at the α-glycol group of phorbol are described. Six of the phorbol diesters synthesized are identical with the irritant and tumorpromoting acetyl-phorbol-acylates A1 — A4, B4 and B7 isolated from croton oil. The isomeric phorbol- (12,13) -diesters A2 and B7 as well as A3 and B4 are pairs of positional isomers regarding the fatty acid residues. It may be generalized for all the eleven phorbol· (12,13) -diesters isolated so far from croton oil that compounds of the A group carry the long-chain fatty acid residue at C-13 and the short chain fatty acid residue at C-12. Compounds of the B group show inverse positions of these fatty acid residues. Isomeric phorbol-diesters of the types A and B may be differentiated by their Rf values in thin layer chromatography, their IR and mass spectra as well as the melting points of their 20- [4'-nitrophenylazobenzoic acid- (4)] - esters. Via the synthetic routes described phorbol- (12,13) -diesters with defined chemical structure are now easily accessible.