The lack of a direct method for measuring activity inside ion-exchamye resins was the main obstacle for strict thermodynamic investigations of such objects during the past decades. This paper proposes a method for the activity determination. The method is based on exploring the ESR response of stable nitroxide radicals introduced as probes into the resin phase. This method allows for the measurement of the hydrogen ion activity inside cross-linked polyelectrolytes. In addition to the method verification, the paper reports its application to the determination of ionization constants of functional groups and to the investigation of the hydrolysis and sorption of copper on different types of sorbents (weak cation and anion-exchange resins, functionalized and nonfunctionalized polymeric films). The applicability of known thermodynamic approaches developed for the investigation of micelles and biomembranes with nitroxide radicals is discussed. The paper also includes a critical analysis of the potentiometric method conventionally used for the investigation of ion-exchange equilibria.