Counterion mixing effects on the volume phase transition and the coil-globule transition of alkali metal poly(acrylate)s (PAAM) in aqueous organic solvents were investigated by observing the swelling behavior of PAAM gel and by measuring the solution viscosity and the conductivity as a function of the counterion mixing ratio. Marked transitions to the collapsed states were induced only for Li+/Cs+ system in most solvent systems; namely, PAA gel significantly collapsed in the presence of Li+ and Cs+ counterions irrespective of the solvent species employed, while only a slight deswelling was observed for Li+/K+ system in some aqueous organic solvents. Corresponding specific decrements in the solution viscosity and conductivity were also confirmed for the combination of Li+ and Cs+ in aqueous dimethyl sulfoxide (DMSO) system. A simple analysis of the conductivity decrement observed for Li+/Cs+ system in 60 vol % DMSO suggested that only Cs+ is tightly bound upon addition of Li+ while no restriction is induced for Li+ upon mixing with Cs+. A working hypothesis is proposed for the apparently intriguing behaviors of the counterions in the mixed system. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2122-2131, 2009