Microcalorimetric titrations have been performed in acidic aqueous solution at 25degreesC to calculate the complex stability constants (K-S) and thermodynamic parameters (DeltaGdegrees, DeltaHdegrees, and TDeltaSdegrees) for the stoichiometric 1:1 complexation of lanthanoid(III) nitrates (La-Gd, Tb) with 5.11,17,23-tetrasulfonato-25.26,27,28-tetrakis(hydroxycarbonylmethoxy)ca lix[4]arene (2) and 5,1 1,17,23-tetrasulfonato-thiacalix[4]arene (3). Using the present and previous reported data on water-soluble calix[4]arenesulfonates (1) and structurally related analogues 2 and 3, the complexation behavior is discussed comparatively from the thermodynamic point of view. Possessing four carboxyls at the lower rim of parent calix[4]arenesulfonate (1), the derivative 2 displays the enhanced binding abilities for Sm3+. As compared with 1 and 2, p-sulfonatothiacalix[4]arene (3) gives not only the lower binding constants for all of lanthanoid(III) ions but also lower cations selectivity. Thermodynamicallv, the resulting complexes of lanthanoid(III) ions with 1 and its derivatives 2 and 3 is absolutely entropy-driven in aqueous solution. typically showing larger positive entropy changes. These larger positive entropy changes (TDeltaSdegrees) and somewhat smaller positive enthalpy changes (DeltaHdegrees) are directly contributed to the complexes stability as a compensative consequence. (C) 2003 Elsevier B.V. All rights reserved.