The catalytic activity of aluminosilicates with respect to the hydrogen redistribution reaction is shown to be due exclusively to exchangeable ions and hydrolytic acidity. There is no important difference between the two kinds of acidity. Silica gel is found to contain active, “hydrolytic,” and nonactive OH groups. Silica gel acquires ion-exchange capacity and catalytic properties exclusively as a result of substitution of aluminum for the protons in hydrolytic OH groups. There is a linear relationship between the content of hydrolytic ions of aluminum and activity. It is shown that H-aluminosilicates are unstable and spontaneously convert by inner exchange to Al-aluminosilicates. Consequently, the exchange acidity of aluminosilicates is due solely to the presence of exchangeable ions of aluminum on their surface. The exchangeable aluminum is a strong Lewis acid. The ability of aluminosilicates to acquire catalytic activity only in the presence of traces of moisture is assumed to be a result of complex formation of the exchangeable aluminum with water. The ionization of this complex leads apparently to the appearance of acid sites of the Brönsted type. Evidentally, the active site is the exchangeable aluminum ion in combination with one water molecule. In catalytic conversion experiments aluminum ions prove to be equal in activity, even though they differ in their ability for exchange in an aqueous medium. The active sites are completely accounted for by the value of hydrolytic acidity. The true value of this acidity may be obtained by the carbonate method. Those methods of titration in anhydrous media by butylamine or ethyl acetate fail to take into account just that part of aprotonic acidity that is responsible for the activity of the catalyst.