Iron and aluminum oxides, which are found in most soils, interact with soil contaminants in a manner depending on soil acidity and water content of the soil. It is important, therefore, to know how acidity and water content affect these oxides. A thermoanalytical study of activated aluminum oxides of three acidities and water concentrations corresponding to Brockmann activity I-V showed that three types of water could be detected on the aluminum oxide, namely free water, pore water, and bound water. Neutral and basic aluminum oxide apparently underwent hydration easily to form the mineral bayerite or gibbsite but acid aluminum oxide required a higher temperature and a longer reaction time for hydration.