The complexation between an anti-cancer drug Daunomycin (DAU) and a Vitamin B-2 derivative, flavin-mononucleotide (FMN), in the presence of a third type of aromatic molecule, caffeine (CAF), in aqueous solution has been studied by NMR spectroscopy. Ternary mixtures of the drug, vitamin and caffeine have been analysed quantitatively taking into account all possible complexation reactions between the aromatic molecules in solution. The results show that complexation between DAU and FMN decreases on addition of CAF which suggests that caffeine at physiological concentrations in vivo may affect the biological synergism of drug and vitamin. (c) 2006 Elsevier B.V. All rights reserved.