The complexation of p-tert-butylcalix[4]arene tetraethanoate, 1a, p-tert-butylcalix[4]arene tetramethyl ketone, 1b, and p-tert-butylcalix[4]arene tetraacetamide, 1c, and trivalent cations was investigated in acetonitrile and N,N-dimethylformamide at 298.15 K using several techniques. H-1 NMR measurements in CD3CN at 298 K were carried out for the systems involving 1a, 1b, and 1c as ligands and Sc3+, Y-3+,Y- Eu3+, and Yb3+ as cations. For the latter ligand, H-1 NMR titration with La3+ was also carried out to assess the sites of interaction of these ligands and the appropriate cation in this solvent. Conductance measurements were performed in acetonitrile and N,N-dimethylformamide with the aim of determining the composition of the metal ion complexes. Stability constants and derived standard Gibbs energies, enthalpies, and entropies reveal that, as far as 1b is concerned, this ligand is not able to distinguish among the trivalent cations as a result of a remarkable enthalpy-entropy compensation effect. This is not the case for 1c and these cations in acetonitrile and to a lesser extent in N,N-dimethylformamide. The selective behavior of this ligand for these metal cations is reflected in the stability constants, which are higher in acetonitrile than in N,N-dimethylformamide. A plot of log K-s values against the cation radius shows a "selectivity" peak. In acetonitrile, the complex stability is greater than that previously observed for an analogous derivative and these cations in this solvent. The medium effect on the complexation process is discussed.