Here, a thermoresponsive phase behavior of polymerized ionic liquids (PILs) composed of poly([tri-n-alkyl(vinylbenzyl) phosphonium] chloride) (poly([P-nnnVB]Cl) is reported, where n (the number of carbon atoms of an alkyl chain) = 4, 5, or 6 after mixing with aqueous sodium chloride solutions. Both monomeric [P-555VB]Cl and the resulting poly([P-555VB]Cl) linear homopolymer show a lower critical solution temperature (LCST)-type phase behavior in aq. NaCl solutions. The phase transition temperature of the PIL shifts to lower value by increasing concentration of NaCl. Also the swelling degree of crosslinked poly([P-555VB]Cl) gel decreases by increasing NaCl concentration, clearly suggesting the "salting-out" effect of NaCl results in a significant dehydration of the poly([P-555VB]Cl) gel. The absorbed water in the PIL gel is desorbed by moderate heating via the LCST behavior, and the absolute absorption/desorption amount is improved by copolymerization of [P-555VB]Cl with more hydrophilic [P-444VB]Cl monomer.