This chapter presents a discussion on standard molar Gibbs energies and activity coefficients of surface complexes on mineral-water interfaces. Adsorption phenomena on of mineral-water interfaces (MWI) govern retention of trace elements and organic substances of environmental concern, as well as the kinetics of mineral dissolution/ precipitation. The success of surface complexation models (SCMs) based on the mole balance constraint for surface sites and on the mass-action expressions for site-binding reactions shows that, for many systems, the non-ideal effects are somehow accounted for, notwithstanding that the activity coefficients of surface species (except the Coulombic term) are usually ignored or assumed to cancel in LMA expressions. The Langmuir isotherm describes the saturation of an adsorbed monolayer by molecules, each strongly sticking to the sorbent surface without any lateral interaction except occupying one site, and without sticking to each other. The chapter also discusses the surface activity coefficient terms (SACT).