A titration technique has been used to directly measure the proton uptake by ion-exchange poly(2-vinylpyridine) coatings as a function of solution pH. Contrary to the common assumption that each nitrogen in the pyridine ring is attached to an H+ when the polymer film is fully protonated, the use of the titration technique has shown that the portion of the pyridine sites actually protonated does not exceed 25% anywhere in the pH range where the coating is stable. The pK(a) value for the poly(2-vinylpyridine) coating has also been estimated to be 4.5 from the titration curve.