The surface of a silica sample (Spherosil) have been modified by means of chemical treatments with sulfuric, phosphoric, and hydrochloric acids and potassium chloride. A sample also has been heated in a furnace at high temperatures. Changes in the degree of hydroxylation have been qualitatively followed by means of IR spectroscopy. Nitrogen, argon, and oxygen physisorption has been employed to study the behavior of the original and treated samples. Monte Carlo computer simulations results have been developed to explain nitrogen adsorption isotherms obtained with the different samples.