The organic acids present are citric, isocitric, and l-malic, with a small residue of unidentified acids. The diurnal variation in acidity is due chiefly to changes, in malic acid, with a parallel fluctuation shown by citric acid. Under these conditions isocitric acid shows little change. The importance of carbon dioxide during acidification is confirmed, and it is shown that at room temperatures or higher the CO2 produced in respiration is sufficient to produce maximum acidification. At lower temperatures the supply of CO2 limits acid production. In the absence of oxygen no acidification occurs, but even small quantities (approx. 1 per cent.) are sufficient to cause some acid production. Complete balance-sheets are presented for acids, carbohydrates, CO2 and oxygen for leaves maintained in the dark at high and low temperatures. As acids are produced there is a corresponding loss of carbohydrate (chiefly starch). A scheme of reactions is suggested to explain the experimental results.