Abstract Protonation constants of polyacrylate (MW = 2 kDa) were determined in several mixed LiCl/KCl aqueous mixtures and mole ratios from y = 0 to 1 y = [Li +] / ([Li +] + [K +]) and I = 1 to 3.5 mol L − 1 . Different models were tested in calculating protonation constants from ISE-H + potentiometric data (namely: Henderson–Hasselbalch, Högfeldt and Dicarboxylic-like models). The function log K H vs. y does not follow an ideal behaviour (Young's rule) but can be expressed by a three parameter equation derived by the zeroth Guggenheim approximation. From this equation it is possible to calculate the free energy of mixing. The dependence on ionic strength of protonation constants was modelled by the SIT (Specific ion Interaction Theory) approach.