The self-association of caffeine and methyl gallate (a simple polyphenol) has been studied by NMR in the solvent mixture H₂O–[²H₆]-dimethylsulfoxide (9 : 1, by volume) at 288 K, using the isodesmic model (also known as the indefinite non-cooperative model). The chemical shift changes observed suggest that caffeine molecules in a stack are associated in a head-to-tail (antiparallel) manner. The interaction between monomeric caffeine and methyl gallate stacks has also been investigated and the results indicate that caffeine prefers to bind at the ends of methyl gallate stacks rather than intercalating into the stacks. Comparison of the association constants suggests that in a mixture of methyl gallate and caffeine, heterostacks are formed in preference to self-associated solutes, which has an obvious bearing on the bioavailability of aromatic and phenolic species.