Calorimetric and potentiometric studies indicate that a cage-type compound, decamethylcucurbit[5]uril (MC5), shows very high selectivity for Pb2+ over alkali, alkaline-earth, NH4+, and Cd2+ cations. The selectivity factor is > 10(5.5), Thermodynamic quantities (log K, Delta H, and Delta S values) for the interactions of MC5 with metal ions were determined at 25 degrees C by calorimetric titration in 50% formic acid/water (v/v) and by potentiometric titration in an aqueous solution. The log K(H2O) value for protonation of MC5 was determined to be 9.56. Pb2+ forms both 1:1 and 2:1 (Pb2+/MC5) complexes in 50% formic acid/water and aqueous solutions, while all other cations studied form only 1:1 complexes with MC5 in the 50% formic acid/water solution. However, both 1:1 and 2:1 complexes have been detected in an aqueous solution for Sr2+, indicating that the stoichiometry of MC5 complexes in 50% formic acid and in aqueous solutions could be different.