The interactions of poly(4-vinyl-N-alkylpyridinium iodides) with methyl orange were studided to compare the hydrophobic effect of alkyl group of the polymers with that of alkyl group of the homologs of methyl orange. The extents of binding between the polymers and methyl orange were measured 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 (ΔS) was largely positive. The longer the alkyl chain of the polymers, the more positive were K_l and ΔH and the larger was ΔS. This result can be explained by the hydrophobic effect of alkyl group of the polymers. The values of K₁ and ΔS for the polymers-methyl orange systems were much higher than those for the poly(4-vinyl-N-methylpyridinium iodide)-the homologs of methyl orange systems. This result can be attributed to the cooperative aggregation of alkyl group of the polymers.