Besides Maillard reactions, structural rearrangements of flavan-3-ol monomers cause color changes in beer during storage. Acetone/water-soluble fractions (70/30, v/v) of three lager beers of the same batch, differently stabilized before bottling in glass or poly(ethylene terephthalate) (PET) bottles, were monitored by normal-phase HPLC-ESI(-)-MS/MS over a 1-year period of storage at 20 degrees C. In parallel, beer color was monitored by the European Brewery Convention assay. The evolution of color was similar in the silica gel-filtered beer to that in identically bottled and stored poly(vinylpolypyrrolidone)-treated samples, despite the high flavanoid dimers content of the former. On the other hand, color evolved more rapidly in the PET bottle, suggesting a key role of oxygen. The kinetics was still increased in model media containing (+)-catechin, while no color was detected when normal-phase HPLC-fractionated dimers or trimers were investigated. (+)-Catechin emerged as the precursor of less polar products, characterized by a yellow-brown color. MS/MS enabled us to identify these products as issued from the oxidation and intramolecular additions of dehydrodicatechin B4. Similar structures were found in aged beers spiked with (+)-catechin. Beer storage in the absence of oxygen and at low temperature is recommended so as to minimize the synthesis of such pigments.