Counterion binding of poly(4-vinyl pyridine) which was quaternized to different extents (10, 15, 20, 25, 30, 35 mol%) was investigated by measuring the apparent counterion activity and the reduced viscosity for the aqueous alcohol (methanol, ethanol and 2-propanol) solutions. Plots of the activity data against the dielectric constant of the solvent suggested that, at a charge density which was close to the critical one for the counterion condensation predicted by Manning theory, the counterions start to be bound to the polyion more strongly than expected by the non-specific condensation mode. At the same critical points, the reduced viscosity started to decrease in spite of an increase in the charge density of the polyion. The polymer concentration dependence of the reduced viscosity suggested that the polycation assumes an ionomer-like conformation in a higher concentration region.