The bindings of nylons with methyl orange were studied, and the first binding constants and thermodynamic parameters for the binding were obtained. It was found that, with increasing the methylene group of the nylons, the free energy for the binding increased negatively and the enthalpy and the entropy increased positively. The favorable free energy change for the binding appeared to be a result of favorable entropy change rather than enthalpy change. All these facts obtained can be interpreted by the hydrophobic interaction between the hydrocarbon moiety of the dye and the hydrocarbon parts of the nylons, and it is reasonable to conclude that not only the electrostatic attraction but also the hydrophobic interaction contributes to the binding of methyl orange, a typical acid dye, by nylons.