Evolution of surface acidity and porosity in montmorillonite clay on treating with sulfuric acid has been studied. It is observed that the clay treated with 4 N sulfuric acid shows the maximum surface acidity. Acid strength distribution as measured by Benesi’s technique of nonaqueous titration shows acidity range mainly between H0 + 4.6 and +3.3. Nitrogen adsorption-desorption hysteresis data indicate the transformation of pores from slit-shaped to spheroidal or ink bottle type as the acid concentration is increased from 1 to 8 N. These observations are explained in terms of structural modifications of clay on treatment with acid.