A novel steady-state shifting technique, which overcomes the limitation of slow molecular diffusion, has been used to extend the applicability of the potentiometric method to highly concentrated and especially supersaturated solutions. The new technique has been verified experimentally and applied to measure the mean ionic activity coefficient ratios of NaCl in ternary glycine + NaCl + water solutions with glycine concentrations ranging from 0.1 m to as high as 4.0 m (i.e. approximately up to a level of relative supersaturation of 20%). New data in the supersaturated glycine region showed interesting indications of significant molecular interactions. Finally, the ternary system data were correlated mathematically to derive the activity coefficient ratios of glycine. (c) 2006 Elsevier Ltd. All rights reserved.