Thermosensitive ionic semi-interpenetrating polymer networks (semi-IPNs) based on poly(N-isopropylacrylamide) (PNIPAAm) and poly(styrene sulfonic acid sodium salt) (PSS) were synthesized, and their properties, such as conversion, swelling ratio and swelling/shrinking kinetics, were studied at different PSS fractions and molecular weight (MW). It is shown that studied semi-IPN hydrogels undergo shrinking under temperature increase. Swelling behavior and volume transition temperature of the semi-IPNs is controlled by the polyelectrolyte fraction. The increase of PSS MW allows obtaining of stable semi-IPN hydrogels. A novel phenomenon of collapse irreversibility in PNIPAAm-PSS semi-IPNs is reported. In contrast to PNIPAAm hydrogels, semi-IPNs of PNIPAAm with immobilized polyelectrolyte do not restore their volume when they reswell after shrinking. The magnitude of this effect is connected to the charged polyelectrolyte fraction. (C) 2012 Elsevier Ltd. All rights reserved.