In this paper, new data are reported on the protonation of phytate at T = 298.15 K in different ionic media and ionic strengths, namely NH4Cl(aq) (0.1 <= I/mol . kg (1) <= 1.9) and NaNO3(aq) (0.1 <= I/mol . kg (1) <= 5.4). A complete set of phytate protonation constants, with general formula H(i)Phy (with 1 <= i <= 7) was proposed. The data were modeled by a logarithmic empiric equation, as well as an Extended Debye-Huckel (EDH) and specific ion interaction theory (SIT) models. A strong similarity was found between protonation data in NH4Cl(aq), NaNO3(aq) and those previously reported in NaCl(aq), KCl(aq) and KNO3(aq), so that an unique set of thermodynamic protonation data has been proposed for phytate protonation at I <= 0.15 mol . kg (1). Thermodynamic parameters obtained in these conditions can be useful to model the behavior of phytate in real aqueous systems at low ionic strengths. An empirical relationship has also been proposed for all the protonation data at 0 < I/mol . kg (1) <= 0.15: T Delta S (+/- 1.5) = 11.7 - 0.72 . Delta G. Using literature data for the phytate protonation in (C2H5)(4)NI(aq), the weak complexes between Na+ or NH4+ and phytic acid have been calculated by the Delta pK method (i.e., by the comparison of the protonation constants determined in an interacting medium, such as NH4Cl(aq), and those obtained in a non-interacting supporting electrolytes, like tetraalkylammonium salts). In terms of weak complex formation constants, the results are similar for NaNO3(aq) and NH4Cl(aq), and twelve H(i)M(j)Phy species are reported together with their stability. For example, for the formation of the M(6)Phy species, it is: log beta = 25.9 and 25.6 (+/- 0.2) for M = Na+ and NH4+, respectively, at I = 0.1 mol . dm (3). This paper represents an advance in the understanding of the acid-base behavior of phytic acid in a wide number of ionic media at different ionic strengths. (C) 2015 Elsevier Ltd. All rights reserved.