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The factors and theoretical approaches to equilibrium swelling of cross-linked polymer systems were analyzed in relation to the application of the equilibrium swelling method for characterization of structure of cross-linked coating films. Special attention was paid to the possibility of characterizing the cross-link density and its changes during film formation. For swelling of highly cross-linked networks (coating films) in good solvents, the finite extensibility of network chains was respected. The effect of adhesion of the film to the substrate was also considered. If equilibrium swelling method (swell test) is used for characterizing changes in cross-link density of drying (reacting) films, the results are often not meaningful for several reasons such as (a) dependence of interaction parameter on conversion; (b) if the sample is in its glassy state and contains unreacted functional groups, increase of conversion occurs during the test when swelling induces transition from the glassy to the rubbery state.