De Boer's t-method and its modifications are convenient versions of a method for reducing adsorption isotherms of nitrogen vapor on adsorbents for which one of the specific surface areas is known. The closer the adsorption isotherms correspond to each other, the better the agreement between the specific surface areas determined by the BET-method ( S BET), de Boer's t-method ( 1) and Sing's α-method ( S α ). For the same adsorption isotherms, of course, S BET = S 1 = S α . In this case, the initial section of the t- or α-curve is expressed by the 2-constant BET equation with closely agreeing constants c for the standard curve and the adsorption isotherm of the adsorbent under study. The theoretical principles of determination of the specific surface areas of adsorbents by the adsorption reduction method proceed from Brunauer's multilayer adsorption theory. The possibility is shown of calculating from an experimental t- or α-curve, a “corrected” curve corresponding to the condition of similarity for the adsorption isotherm under consideration and leading to a quantitative agreement between S BET , S t , and S α .