The basic principle of the t- and α s-methods is to compare the adsorption isotherm under test to a standard isotherm in a reduced form, corresponding to a nonporous reference solid. It is demonstrated that the standard isotherm has to be chosen according to the intensity of the adsorbentadsorbate interactions, expressed by the C BET constant. A set of experimental reference isotherms, corresponding to definite ranges of the C BET constant value between 20 and about 3000, is proposed. These isotherms are shown to be very well fitted by the n-layers-BET equation, with n nearly equal to 4. It is also shown that the t- and α s-methods are strictly relevant to the BET theory and that these two methods are two equivalent ways of analyzing adsorption isotherms. In particular, the microporosity of solids can be assessed, within the limits of the BET model, provided that the reference isotherm is correctly chosen. All these results are proved to be valid whatever the nature of the adsorbent-adsorbate pair used. Some isotherms obtained with N 2, Ar, CO, O 2, CO 2 are reported.