Binding isotherms of the dodecylpyridinum ion to copolymer gels of methyl acrylate/acrylate (MA), ethyl acrylate/acrylate (EA), propyl acrylate/acrylate (PA), and n-butyl acrylate/acrylate (BA) were compared to examine the effect of the hydrophobicity of the gel chain on the binding behavior of hydrophobic counterions. The cooperativity of the binding decreased when lengthening the chain of alkyl acrylate. In the binding to the BA gel, anticooperative binding was observed. The binding constant increased with the chain length of alkyl acrylate. Linear dependencies on the alkyl number of alkyl acrylate were found for both of the logarithm of observed cooperativity parameters and binding constants. The dependency of the gel volume on the degree of the binding for BA gels was found to be significantly different from those for MA, EA, and PA gels.