The contribution of hydrophobic interaction to the binding of nylons with methyl orange and its homologs was investigated from the thermodynamic analysis of the binding system. It was found that the free energy increased negatively with the hydrophobicity of dye and nylon against unfavorable enthalpy situation in the binding system, which means that the binding is entropy-driven. The contribution of hydrophobic interaction to the binding was much pronounced in the binding of nylon 612 with butyl orange where the free energy increases negatively in spite of endothermic enthalpy situation. These results show that hydrophobic interaction contributes significantly to the binding of nylon with acid dye, and that not only hydrophobicity of nylon but also that of dye affects the hydrophobic interaction