<p>Non-additive behavior in multicomponent systems is commonly described using empirical indices that lack direct thermodynamic interpretation. In this work, a general thermodynamic criterion is developed for identifying and quantifying non-additive behavior in systems with coupled equilibria. The formulation is based on a measurable deviation parameter defined as the ratio between real and pairwise-additive reference states and leads to the definition of an irreducible contribution to excess Gibbs free energy. A thermodynamic relation is derived showing that deviations from pairwise-additive behavior can be quantified through a measurable deviation functional and its corresponding Gibbs-energy contribution. The formulation is derived from general thermodynamic considerations and does not depend on the detailed chemical identity of the interacting species, provided that the real and reference states are defined consistently. The implications of the criterion are illustrated using a representative multicomponent system involving coupled homogeneous and heterogeneous equilibria. The analysis shows that non-additive behavior becomes pronounced near phase boundaries and can be expressed as an irreducible Gibbs-energy contribution. This relation allows non-additivity in complex chemical systems to be quantified from equilibrium observables.</p>