The rats of thermal ageing at elevated temperatures and heats of activation of aluminum hydroxide gel have been determined. The ageing was expressed in terms of diminution of acid consuming capacity. Although the actual rates of ageing were different by the samples, the slopes of their Arrhenius plots were the same. This may indicate similarity of ageing mechanisms and heats of activation of samples. The heat of activation was the mangitude of 30 Kcal/mole. Thus, rates of ageing of aluminum hydroxide gel may be greatly accelerated at elevated temperatures.