Copolymerization of 2-acrylamido-2-methylpropane sulfonic acid (AMPS, monomer I)with 2-hydropropyl methacrylate (HPM, monomer 2) was conducted in ethylene glycol/water (1 : 1 in weight) at 70 degrees C. The reactivity ratios estimated from the copolymer composition at low conversion are r(1) = 2.31 +/- 0.25 and r(2) = 11.70 +/- 1.05. The azeotropic composition was found at the monomer mole ratio AMPS/HPM equal to 8/2. Viscosity of these copolymers was measured in dimethyl sulfoxide (DMSO) and DMSO/tetrahydrofuran (THF) mixed solvent at 25 +/- 0.05 degrees C. Polyeletrolyte behavior was observed for all the copolymers, even in the mixed solvent containing 65 wt % of THF. The reduced viscosity at constant polymer concentration decreased with increasing THF content in the mixed solvent. The copolymers having AMPS repeat units more than 42 mol % precipitated in the mixed solvent when the THF was beyond 68 wt %. The viscosity reduction and precipitation in the copolymer solutions with increasing THF can be attributed to the dipole-dipole attraction between ion-pairs formed in less-polar medium. This is helpful in understanding the volume phase transition in highly charged hydrogels caused by mixing solvents.