The pH inside a swollen polyelectrolyte network is calculated through a simple model, based on Donnan equilibrium and balance of mobile ions, extended to include the presence of a supporting electrolyte (salt) in the solution that swells the particle. This pH inside the gel,although needed to characterize the ionization properties of the polyelectrolyte, is generally not accessible to direct measurement. The main advantage of our model is that it is free from any simplification concerning the pK(a) of the ionizable groups. A common univalent anion is assumed for the acid and salt. The model was applied to chemically cross-linked poly(N-vinylimidazole) (PVI) immersed in acidic aqueous baths containing variable concentrations of HCl and NaCl as Supporting electrolyte. The imidazole units are basic and become protonated by the acid, thus changing the pH of the initial bath. The data needed for the calculation of the proton concentration inside the gel, the degree of ionization, and the pK(a) of the polyelectrolyte are: polymer concentration. pH and salt concentration in the initial Solution, and pH in the bath at equilibrium. All of them can be determined experimentally by a batch method, where the polymer is immersed in a different pot for each starting pH and salt concentration. It was thus found that in salt free solutions the pH inside the gel is several units higher than the pH in the external bath at equilibrium, but this difference between internal and external pH faints with added salt. The intrinsic pK(a) of PVI, determined from the pH in the gel, is slightly higher than the pK(a) of the model molecule, for salt free solutions, but it is lower with added salt (possibly due to the formation of a hydrogen bond between two imidazole units and its disruption by chloride). It was concluded that the pH inside the polymer must be employed instead of the pH Outside, in order to calculate pK(a), not only for a swollen polymer network, but also for a dissolved coil.