The results of an investigation on the interactions between phytate ion (myo-inositol hexaphosphate, Phy) and some lanthanoid cations (La3+, Nd3+, Sm3+, Dy3+, and Yb3+) are reported. The stability constants of various LnH j Phy species (Ln = generic lanthanoid) were determined by potentiometry (ISE-H+ glass electrode) in NaClaq at I = 0.15 mol dm−3 and t = 25 °C, and the corresponding formation enthalpies by calorimetric titration. The thermodynamic data obtained were used to provide a speciation scheme for the lanthanoid(III)–phytate systems at different temperatures. The sequestering ability of this ligand toward Ln3+ was also evaluated by calculation of pL50 values (the total concentration of ligand necessary to bind 50% of a cation present in trace amounts) under different conditions, and equations were formulated to model their dependence on temperature and pH.