A polyion complex was formed by mixing poly(methacrylic acid) (PMA) and poly(4-vinylpyridine) (P4VP) in methanol. Its structure and composition were examined by means of infrared spectroscopy(IR) and elemental analysis. Excess of PMA in the product was observed when concentrated solution were used or when an excess of PMA was added to P4VP. The binding of methyl orange and its homologs by PMA/P4VP complex was examined at 20-4$0^{circ}C$ temperature range. The first binding constants and the thermodynamic parameters in the course of binding were calculated. It was found that the free energy and the enthalpy changes are negative and the entropy change is positive. The longer the alkyl chain of the dyes is the larger the entropy change is. The negative values of$Delta$G increase with increasing temperature. These characteristics of the thermodynamic parameters suggest that hydrophobic interaction between dye and complex plays an important role in binding. In relation to this hydrophobic interaction, the effects of urea and KCl on the binding of methyl orange were also investigated.