Acylation with hydroxycinnamic acids stabilizes anthocyanins and makes their colour bluer (bathochromic shift). We purified to homogeneity one acylation enzyme, hydroxycinnamoyl-CoA:anthocyanidin 3,5-diglucoside 5-O-glucoside-6"'-O-hydroxycinnamoyltransferase, from blue petals of Gentiana triflora. It is a single polypeptide protein of 52 kDa with a pI of 4.6. It catalyzes the transfer of the p-coumaric acid and caffeic acid from their CoA esters to the 5-glucosyl moiety of anthocyanidin 3,5-diglucosides but could not use malonyl-CoA as an acyl donor. Neither anthocyanidin 3-monoglucoside nor anthocyanins aromatically acylated at the 3-glucosyl moiety could be acylated by this enzyme. Aromatic acylation of anthocyanidin 3,5-diglucoside by this enzyme caused a bathochromic shift and increased pigment stability in neutral to weakly basic pH. Other anthocyanins from the petals of G. triflora were isolated and their structures were determined by fast-atom-bombardment MS and NMR. The biosynthetic pathway of gentiodelphin, a diacylated anthocyanin accumulating in G. triflora petals, is proposed on the basis of these results.