Acid-base properties of phytic acid [1,2,3,4,5,6-hexakis(dihydrogen phosphate)-myo-inositol] in aqueous solution have been studied, by potentiometric measurements ( [H(+)]-glass electrode), in tetraethylammonium iodide (Et(4)NI) and sodium chloride aqueous media, at different ionic strengths (0 < I mol L(-1) less than or equal to 1 for Et(4)NI and 0 < I mol L(-1) less than or equal to 5 for NaCl) and at t = 25 degreesC. Protonation of phytate ion proved strongly dependent on both ionic medium and ionic strength. The protonation constants obtained in NaCl are considerably lower than the corresponding ones in Et(4)NI (e.g., at I = 0.5 mol.L(-1), log K(3)(H) = 11.7 and 9.1 in Et(4)NI and in NaCl, respectively), owing to the strong interactions occurring between phytate and sodium cations present in the background salt. Some empirical equations are proposed for the dependence of protonation constants on ionic strength for both ionic media. Also, the function log K(i)(H) versus i (i = protonation step) can be modeled, and this allowed us to guess some protonation constants (in Et(4)NI) at very high pH values.