The characteristic ionization of poly/vinylamine) (PVAm) in aqueous solutions under various conditions has been evaluated precisely by conductometric and pH-metric measurements and by measuring the visible light absorbance by a small amount of the pH indicator Bromophenol Blue (BPB), which coexisted in the solution. The experimental data obtained by these different methods, were analyzed by the nonlinear Poisson-Boltzmann cell model, which takes the dissociation of water into account. As a result, the dissociation equilibrium between protons and amino groups at the PVAm surface was proved to follow the one-dimensional Ising statistics with the interaction parameter 3.9 and the intrinsic pK(a) of amino groups 10.0. Compared with poly(allylamine hydrochloride) (PAAmHCl) in their conductance variations of the solutions as neutralized with NaOH, the strong repulsive interaction between protons and ammonium groups of polyamines turned out to be largely influenced by the existence of a methylene spacer between amino residues and the main chain.