Binding isotherms of Ca2+ ta copolymer gels of acrylamide and acrylate (AA), N-isopropylacrylamide and acrylate (NA), and n-butylacrylate and acrylate (BA) were investigated and were compared with each other for examining the effect of the hydrophobicity of chain on the Ca2+ binding, The highest binding constant and the largest shrinkage in the volume were observed for the BA gel. The counterion condensation theory developed by Manning could not explain the observed binding isotherms. The analysis using the threory for the cooperative binding developed by McGhee and Hippel indicates that the binding to BA gel is more cooperative than those to the others, The present experimental result indicates that the hydrophobic circumstance promotes the Ca2+ binding to carboxylate.