The interaction of poly (4-vinyl-N-methylpyridinium iodide) with methyl, ethyl and propyl orange was studied by an equilibrium dialysis method at various temperatures. The first binding constants (K₁) and the thermodynamic parameters for the binding systems were calculated. It was found that the free energy change (ΔG) and the enthalpy change (ΔH) were all negative and the entropy change (△5) was largely positive. The longer the alkyl chain of the dyes, the more positive were K₁ and ΔH and lienee the larger was ΔS. The favorable ΔG for the binding systems seemed to be a result of ΔS rather than ΔH. Temperature dependences of the thermodynamic parameters were apparently observed. That is, ΔG and ΔH became larger in absolute magnitude as the temperature increased. The positive quantity of ΔS tended to decrease with increasing temperature. These results obtained could be interpreted by the hydrophobic interaction between alkyl groups of the dyes and poly (4-vinyl-N-methylpyridinium iodide).