The values of the nonideality factor, alpha, obtained by P. H. Emmett and S. Brunauer, (J. Am. Chem. Soc. 59, 1553 (1937)) with the aid of the van der Waals equation of state for various adsorbates, and which are routinely utilized in adsorption experiments, are valid only for nonpolar gases such as nitrogen, argon, and oxygen, On the other hand, for most gases, the alpha’s from three-parameter equations of state, such as the virial equation involving the correlation of Fitter (PIT) and the Peng-Robinson (P-R) equation, are in excellent conformity with those retrieved from experimentally measured P-V-T data. The PIT correlation is the preferred choice for nonpolar and partially polar organic and inorganic adsorbates on account of its accuracy and amenability to simple numerical calculations. The P-R equation, although complex, is suited for predicting the alpha’s of highly polar gases at various temperatures of adsorption, A new set of alpha’s recovered from these two equations for various adsorbates will potentially facilitate adsorption and chemisorption measurements in vacuum volumetric systems.