Poly(N-vinylimidazole) hydrogels immersed in aqueous acid solutions produce an increment in the pH of the bath because of proton uptake by basic imidazole moieties, leading to hydrogel protonation. Both kinetic and equilibrium measurements of the pH of the bath have been performed under a variety of conditions and with different hydrogel samples. The kinetics of the xerogel protonation process (which includes solvent and titrant diffusion, the true protonation reaction or ion-dipole association, and the polymer relaxation to a new conformation) are mostly driven by the size of the hydrogel sample, whereas other magnitudes, such as the initial pH, the effective polymer concentration, and the network structure, governed by the crosslinker ratio and total comonomer concentration in the feeding, have a minor influence. pK(a) changes with the degree of protonation (a), delimitating two different regions: (1) a broad a range in which pKa decreases with increasing a but less pronouncedly with increasing ionic strength and (2) an a range close to alpha = 1 in which pK(a) decreases abruptly, more markedly with sulfate than with chloride counteranions and with larger ionic strengths. In the first region, pKa is determined by repulsive electrostatic interactions and so is larger for titration with H2SO4 than with HCl and increases as the effective polymer concentration and ionic strength increase. Two steps (i.e., two protonation sites) can be observed in the titration curves, the second. one corresponding to abrupt changes in the basicity of the second pK(a)-versus-alpha region. (C) 2004 Wiley Periodicals, Inc.