Poly(ionic liquid)s (PILs) are a subclass of ionic liquid that feature polymerizable groups either in the cation, the anion or both. Applications of these materials include solid ion conductors, CO2 absorption and energy storage. Furthermore, a branch of PILs feature lower critical solution temperature (LCST) behaviour, making them suitable for the synthesis of temperature responsive materials. The aim of this study was to synthesize thermo-responsive crosslinked PIL hydrogels based on phosphonium PILs. Moreover, taking into account that these PIL hydrogels consist of polyelectrolyte chains, the effect of added salt was also investigated.