This study presents a general adsorption isotherm for gases and gas mixtures which separates in rigorous fashion effects due to adsorbate size r(i), loss of symmetry or chemical dissociation s(i), clustering m(i) and molecular interactions in the adsorbate phase W-ij. The isotherm reduces, in various limiting forms, to the isotherms of Langmuir, Freundlich, Bragg-Williams, Honig (1966, in The Solid-Gas Interface, Vol. 1, Marcel Decker, NY), Sips, Nitta et al. (1984, J. Chem; Engng. Japan 17, 39-52) and others, and agrees with the Fowler-Guggenheim expression for the dissociative sorption of H-2. Comparison with experimental data ranges from the mildly non-ideal involving size effects only, to highly non-ideal mixtures. The Flory form of the general isotherm is y(i)P = M(i) theta(i)(st/mt)/(1-Theta)(rt) exp (r(i) (j=1)Sigma(c) W-ij/kT theta(j)). The isotherm can be cast in straightforward fashion into a Raoult’s law form which has the virtue of separating non-idealities due to size effects, loss of symmetry and clustering psi(i) from those due to molecular interactions in the adsorbate phase beta(i). y(i)P = P-i*chi(i) psi(i) beta(i). Here the vapour pressure of pure component iP(i)* is the pressure it would have at the same temperature and surface coverage of the mixture. In the limit of ideal mixed Langmuir adsorption the treatment reduces to the IAST of Myers and Prausnitz (1965, A. I. Ch. E. J. 11, 121-127).