The effect of different inorganic salts on aqueous solution viscosifying behavior of a thermoassociative water-soluble terpolymer, poly(acrylamide-co-2-acrylamido-2-methylpropane sulfonic acid)-g-poly[acrylamide-co-N-(1,1-dimethyl-3-oxobutyl)acrylamide] (PADAS) was examined in comparison with a nonthermoassociative sulfonated copolymer, poly(acrylamide-co-2-acrylamido-2-methylpropane sulfonic acid) (PAMS). With increasing salt concentration, thermoviscosifying ability of PADAS was clearly evidenced and the magnitude of thermoviscosifying effect was largely enhanced. The ability of cations and anions to enhance the viscosity of PADAS aqueous solution follows the orders Na+ > K+ > Mg2+ > Ca2+ and SO42- > Cl- > Br- > NO3-, respectively. Under the same conditions, PAMS shows only thermothinning behavior and viscosity loss. The thermoviscosifying capability of PADAS is relevant to the change in the polarity of aqueous solution and the electrostatic shield effect of counterion for the repulsion between the sulfonate anions on the polymer, and depends mainly on the competition of these two effects. Cryo-TEM observation demonstrates that the microstructure changes of polymers in the electrolyte solution lead to the changes of the viscosifying behavior. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012