During the maturation phase of red wines, extending from the end of vinification until bottling, the presence of oxygen provokes the chemical transformation of pigments, essential for ageing. Storage conditions (oak wood barrels or large tanks) should include a dissolution of oxygen of about 40 mg litre−1 during 1 year. Under winery or cellar conditions of storage, the autoxidation of ethanol, in the presence of phenolic compounds, produces traces of acetaldehyde. Simultaneously, this acetaldehyde provokes a copolymerisation of anthocyanins and tannins; in the condensed form, the anthocyanins are more coloured than in the free form. Hence aeration provokes an increase in colour, even though concentration of anthocyanins decreases as determined by chemical analysis. As condensation reaches a certain level, the large molecules precipitate and colour decreases. The same oxidation produces an increase in the level of tannin condensation and consequently a decrease in astringency, favourable from the point of view of quality. As a whole, the rate of these condensation reactions is reduced by an excess of SO2.