In this paper we describe the heterophase model of a swollen particle of cross-linked polyelectrolyte in water. We also propose a new definition of a solution of cross-linked polyelectrolyte. We demonstrate that the activity of water in a solution of cross-linked polyelectrolyte depends on two independent parameters: the number of cross-links in the solution of the polyelectrolyte and the activity of water in the equilibrium state. Using a three-dimensional coordinate system, we illustrate the effect of these parameters on the amount and activity of water in the solution of cross-linked polyelectrolyte. The equilibrium between a solution of low-molecular-mass electrolyte and a solution of cross-linked polyelectrolyte is described from the standpoint of classical thermodynamics. We also present methods to calculate amounts and activities of components in the solution of cross-linked polyelectrolyte. In order to describe a solution of cross-linked polyelectrolyte, in addition to the concept of ＇isotherm of sorption of water＇, it is necessary to introduce and define the term ＇isotherm of sorption of electrolyte＇. Examples of such isotherms are provided as an illustration. We propose methodologies to experimentally determine several parameters of the model and present the results of calculations of amounts and activities of the components in a solution of cross-linked polyelectrolyte.