Phase equilibria for solvent mixtures and strong acidic ion exchange resins in H+ form are investigated. Experimental data on ternary non-reactive solvent-solvent-polymer systems as well as reactive multicomponent systems are presented for moderately and highly cross-linked poly(styrene-co-divinylbenzene) (PS-DVB) resins. Esterification of acetic acid with ethanol is used as a model reaction. The data are correlated with a combination of thermodynamic models derived for polymer solutions and gels. Independently determined data is used whenever possible with a goal of reducing cross-correlations between the model parameters. The limitations of the thermodynamic modeling approach for solvent-ion exchange resin systems are discussed. It is shown that, due to glass transition of the polymer matrix, the underlying assumptions are not entirely valid in low dielectric constant media and at high cross-link densities. (C) 2004 Elsevier B.V. All rights reserved.