It is generally necessary to describe a measured gas/solid isotherm with an equation including two or three parameters descriptive of the physical properties of the adsorbent or adsorbate. For this reason it is very important to select the appropriate isotherm equation. The fitting procedures alone are not sufficient for this purpose. The function psi(p) is proposed for this aim. This function can be calculated from the measured isotherms because, based on the implicit form, psi r(p) = (n(s)/p)(dp/dn(s)), a PC can be used to calculate an explicit result. The explicit function psi(p) provides exact information on: (i) the thermodynamically and mathematically appropriate isotherm equation; (ii) the mono- or multilayer domains of the measured isotherms; (iii) the change in free energy of the surface; and (iv) the specific surface area of the adsorbent. These calculations can be summarized in software, so it is proposed that this be applied to adsorption experiments.