Uptake of salts by insoluble polyelectrolyte complexes (PECs) leads to plasticization, and here it is shown that ionic liquids (ILs) are more effective plasticizers than simple organic salts such as NaCl. The PEC uptake of IL cation was monitored by solution H-1 NMR, and the mechanical impacts of plasticization were tracked by dynamic mechanical analysis (DMA). PECs prepared with polystyrene sulfonate (PSS) and poly(diallyldimethylammonium chloride) (PDDA) under charge stoichiometric conditions were immersed in aqueous solutions of 1-butyl-3-methylimidazolium chloride [BMIM][Cl] to cause IL uptake, which could be controlled by the solutions IL concentration: higher concentration leads to higher uptake which leads to greater plasticization. The effectiveness of plasticization was assessed through the position and height of a DMA tan(delta) peak ascribed to a glassy-to-rubbery PEC transition. Consistent with greater PEC uptake, isothermal titration calorimetry demonstrated that solution binding by PSS of [BMIM]+ was much stronger than binding of Na+.