Counterion binding in Na poly(acrylate) gel immersed in water/organic solvent [ethanol (EtOH), acetonitrile (AcN), or tetrahydrofuran (THF)] mixtures was investigated by Na-23-NMR spectroscopy. With an increase in the content of an organic solvent (similar to40-50 vol %), the Na-23 chemical shift significantly moved downfield on a gel collapse. This downfield shift strongly suggests that the gel collapse was induced by contact ion-pair formation between the counterion and the carboxyl anion on the polymer. With a further increase in the solvent content (similar to90 vol %), the chemical shift for an EtOH system showed a slight upfield shift, while THF and AcN systems maintained downfield shifts. The contrasting behaviors for EtOH and the latter two solvent systems were interpreted as being caused by desolvation and resolvation of bound Na+ counterions because of deswelling and reswelling of the respective gels in the pertinent solvent concentration regions. (C) 2004 Wiley Periodicals, Inc.