The formation of metal ion complexes ML (n-1)+ and ML 2 (n-2)+ of the oxygen tripod ligand L - = cyclopentadienyltris(diethylphosphito-P)cobaltate(l-) with metal ions M n+ = Li + , Na + , K + , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ,Cd 2+ , Hg 2+ , Pb 2+ , Ag + ,and La 3+ has been studied by pH titration at 25 °C in 95/5 methanol/water solutions kept at an ionic strength of 0.1 by means of tetramethylammonium chloride. The stability of the alkali metal ion complexes increases with decreasing metal ion radii. The sodium complex NaL is more stable than the known crown ether sodium complexes. The K 1 values of the transition metal complexes ML+ are ≈ 10 9 [1/mol]. For H + , Na + , Mg 2+ , and La 3+ the enthalpies and entropies of complex formation have been determined by calorimetric measurements. The results are comparable to those of the tripolyphosphate anion. The complex formation is slightly endothermic and the stability of the complexes arises from a large positive ⊿S of reaction.