Fluorescence intensities of poly(2-vinylpyridine) (P2VP) and poly(4-vinylpyridine) (P4VP) in H 2 SO 4 /H 2 O solutions were increased with increasing acid concentration. The intensities for P2VP were found to be six times stronger than that of P4VP. These differences were accounted for by the microenvironment of protonated pyridinium group. The ion binding properties of 4-methylpyridine (4MP), P2VP, and P4VP were investigated in methanol using Tb 3ϩ as a fluorescence probe. The increase of fluorescence intensity of Tb 3ϩ in [P2VP-Tb 3ϩ ] and [P4VP-Tb 3ϩ ] complexes is due to both the replacement of the inner coordinated methanol molecules and ligand-to-metal energy transfer. The model compound 4MP was inefficient from this point of view, and the results were attributed to the polymer cooperative effect. Reduced viscosities of poly(vinylpyridine)s (PVP) in methanol were similar to nonionic polymers; however, when TbCl 3 was added into the solution, the viscosities increased upon dilution. These results also indicated that PVP form complexes with Tb 3ϩ in methanol. When diluted, the counterions Cl Ϫ are allowed to dissociate and the charged polymer expands. Consequently, the solution's viscosity increases.