Distribution data for alkali metal picrates in the water-chloroform solvent system in the absence and in the presence of acetonitrile were used to determine the partition constants of the dissociated electrolytes in the mutually saturated solvents and the ion-pair formation constants in the water saturated organic phase. These data were compared with those previously reported in water-benzonitrile and water-dichloromethane solvent systems. The same experiments carried out in the presence of ethyl p-tert-butylcalix(4)arene tetraethanoate, EtCalix(4), led to the calculation of distribution and extraction constants in these solvent systems. The presence of acetonitrile in the organic phase enhances significantly the extraction constant to an extent that these are the highest values so far obtained in the solvent systems investigated. This enhancement in extraction is attributed to metal complex stabilization promoted by the presence of acetonitrile in the hydrophobic cavity of the ligand. The X-ray structures of two sodium complexes (picrate as the counter-ion), isolated from water saturated chloroform with and without acetonitrile are reported for the first time. In the first complex, the macro-cycle is sited on a crystallographic four-fold axis and hosts an acetonitrile solvent molecule in its calix and a sodium ion in its hydrophilic cavity. The metal is in a eight-fold Archimedean square anti-prism environment, coordinated to the phenol oxygen atom and to the carbonyl oxygen atom of the four rotationally symmetry related pendant arms. The macro-cycle of the other complex adopts a slightly distorted cone conformation and its hydrophilic cavity also hosts a sodium ion. But now the metal is in a seven-fold environment, coordinated to the four phenol oxygen atoms and to the carbonyl oxygen atoms of three pendant arms acting as bidentate ligands. The fourth, mono-dentate, pendant arm points away from the hydrophilic cavity and its terminal -CH 2-CH3 group interacts hydrophobically with the calix of a neighboring complex giving rise to a supra-molecular polymeric structure in the lattice.