An amphiphilic electrolyte, methacryloxyethyl dimethyloctane ammonium bromide (MODAB), was synthesized by quaternization of dimethylaminoethyl methacrylate (DMAENU) with 1-bromooctane. Then amphiphilic PMODAB gel was synthesized by radiation cross-linking with ethyleneglycol dimethacrylate as cross-linker. PMODAB gels possess distinguished different swelling behaviors with those of DMAEMA hydrogels: (1) Antipolyelectrolyte phenomenon was observed in NaCl solution, that is, a remarkable increment of equilibrium degree of swelling (EDS) in the low NaCl concentration region (10(-4) to 10(-3) mol L-1), followed by a significant decrease (10(-3) to 0.2 mol L-1), and a collapsed state (>0.2 mol L-1); (2) Compared with PDMAEMA, PMODAB was a temperature-sensitive polymer which showed an upper critical solution temperature (UCST) behavior. The EDS of PMODAB gel dramatically increased at the UCST; (3) In the 0.2 mol L-1 NaCl solutions, PMODAB gel swelled more significantly in a basic condition (pH > 10) than that in an acidic condition. The special volume phase transition behavior of PMODAB gel is ascribed to the hydrophobic interaction between octyl groups and the formation of ion-cluster between tetraalkyl ammonium cation and Br-, which lead to the aggregation of gel structure and could be affected by the composition and temperature of the surrounding solution. (C) 2007 Wiley Periodicals, Inc.