The influence of single salts (either NaCl, K2CO3, KI, KSCN, KF and Na2SO4) on the degree of swelling of some copolymeric hydrogels of N-isopropyl acrylamide (N-IPAAm) and a zwitterionic comonomer sulfobetaine (=[3-(methacryloylamino) propyl] dimethyl (3-sulfopropyl) ammonium hydroxide inner salt (MPSA)) in water at 298 K was investigated experimentally. As expected, the experimental results reveal that at low salt concentrations (mass fractions of salt in aqueous solution below about 10(-3)) none of the investigated salts has an influence on the degree of swelling. As also expected, at high salt concentrations (mass fraction above approximate to 10(-1)) the gels collapse. However, an increase of the degree of swelling at intermediate salt concentrations was found for all investigated gels in aqueous solutions of either KSCN or KI as well as for the gels with a high content of sulfobetaine in aqueous solutions of NaCl. This phenomenon is called "anti-polyelectrolyte" behavior. A previously developed thermodynamic model was extended to describe (i.e., correlate and predict) the experimental results for the swelling behavior. Calculation results for the degree of swelling are in good agreement with experimental data. In most cases the difference between the experimental and the calculation result for the degree of swelling is lower than the corresponding experimental uncertainty. (C) 2014 Elsevier B.V. All rights reserved.