Reactions of seed and skin proanthocyanidins in the presence or absence of (-)-epicatechin were followed in winelike solutions over 53 days at 30 degrees C. Proanthocyanidins were separated from flavanol monomers by sequential elution from a Sep Pak cartridge, and changes in proanthocyanidin composition were monitored by thiolysis analysis of the proanthocyanidin fraction. In solutions containing no free (-)-epicatechin, trace amounts of monomers were released and important losses of proanthocyanidins were measured, but their average composition and mean degree of polymerization (mDP) were hardly modified. In the presence of (-)-epicatechin, the mDP value decreased and oligomeric proanthocyanidins accumulated throughout the incubation while losses of total units were dramatically reduced. Our data indicate that interflavanic bond cleavage of proanthocyanidins occurred under mild acidic conditions such as encountered in wine and that the resulting carbocation proceeded to unknown species. The latter reaction did not take place in the presence of (-)-epicatechin. Epicatechin added to the intermediate carbocation, thus being incorporated as the end unit of a shorter proanthocyanidin chain. The results of this study are discussed in relation to the loss of astringency reported during wine aging.