It is proposed that a fundamental deficiency in the existing theoretical foundation of surface adsorption is that adsorption density (Gamma) is incorrectly used as a state variable. A metastable-equilibrium inequality is obtained when Gamma is not treated as a state variable. The inequality indicates that experimentally measured "equilibrium" adsorption constants for a given adsorption system decrease as the actual metastable-equilibrium adsorption states deviate from the ideal equilibrium state. Isotherms, when expressed in terms of Gamma, can therefore be fundamentally influenced by the kinetics of the adsorption process. This implies that previously measured equilibrium adsorption data may show a lack of consistency, thereby making the use and comparison of the data problematic. The inequality reveals a simple relationship between the real metastable-equilibrium constant and the ideal equilibrium constant, which provides a possible way to modify some of the existing equilibrium theories into metastable-equilibrium theories. As an application of the method, Langmuir-type and Freundlich-type metastable-equilibrium isotherm equations are obtained which reveal that the adsorption isotherm declines as the reversibility of the adsorption process decreases. Based on this concept, it is proposed that particle concentration (C-p) can universally affect adsorption isotherms, depending on its effect on the metastable-equilibrium adsorption state or the adsorption reversibility.