The stability of M/Phy species (M = Ag+, CH3Hg+, Mn2+, Fe2+, Co2+, VO2+, and Phy = Phytate) was determined in NaNO3, at I = 0.10 mol.dm(-3) and T = 298.15 K, potentiometrically. For the VO2+/Phy system, the measurements were performed at different ionic strengths (to 0.45 mol.dm(-3)) and ionic media (NaNO3 and Na2SO4). In general, the formation of six MH(q)Phy (with 0 <= q <= 5) species was observed. For the CH3Hg+/Phy system, the formation of three polynuclear species, M(2)H(q)Phy (with 2 <= q <= 4), was also evidenced. The general stability trend is: VO2+ similar to CH3Hg+ > Co2+ > Mn2+ similar to Fe2+ > Ag+. The sequestering ability of phytate toward the considered metal cations was evaluated at different pH values and increased with increasing pH, except for methylmercury and vanadyl. Literature values were critically compared, where possible, with data reported in this work. The dependence of the formation constants of the MHqPhy species on the number of protons and on the first metal hydrolysis constants was modeled. Furthermore, a relationship between the stability of the mononuclear MH(q)Phy and the polynuclear M(p)H(q)Phy species was found. These three relationships were then combined, for testing and predictive purposes. The last contribution of this research group to the series entitled "Speciation of phytate ion in aqueous solution" is reported in ref 1.