The influence of water activity on swelling of cross-linked hydrophilic polymers in electrolyte solutions of various concentrations was studied. A heterophase model was proposed to explain the polymer swelling. It was shown that the change in polymer swelling deals with different water sorption, and is described by water vapour sorption isotherm. At the same time, the volume of external solution taken up by polymer at swelling is constant and independent of concentration of the solution. Three models describing the structure of polymer gels were suggested and discussed on the base of the obtained results. (C) 2013 Elsevier Ltd. All rights reserved.