The adsorption of water at 30 °C was measured on two Egyptian clay samples. The difference in their adsorption capacities was attributed to more than one factor. The surface area as measured by nitrogen adsorption decreases with acid treatment of the clays. The acidity and catalytic activity increase with the severity of acid treatment. Not all the hydroxyl groups are responsible for catalytic activity and some of them are not acidic. The catalytic cracking activity depends to some extent on the number of hydroxyl groups but to a greater extent on the number of acidic groups per unit surface area.