Abstract 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.