The complexation of the sodium cation by a calix[4]arene tetraester, 5,11,17,23-tetra-p-tert-butyl-25,26,27, 28-tetrakis((ethoxycarbonyl)methoxy)cal (1) was studied by H-1 and Na-23 NMR in a 50:50 v:v mixture of deuteriated acetonitrile and deuteriated chloroform. A 1:1 complex was formed, in very slow exchange, both on the H-1 and on the Na-23 NMR time scales, with solvated calixarene and sodium cation, respectively. From H-1 NMR 2D exchange spectroscopy experiments and Na-23 NMR line width analysis, an upper limit of the rate constant for the dissociation of the 1:1 complex was estimated to be 3 Hz. A 2:1 calixarene:sodium complex in fast exchange with the 1:1 complex was evidenced by Na-23 NMR. The characteristic Na-23 NMR line widths of the 1:1 and of the 2:1 complexes, determined at several temperatures, were linearly related, with a slope of 1.8 +/- 0.2. This corresponds to a reorientational correlation time, characteristic of the Na-23 quadrupolar relaxation, twice as large in the case of the 2:1 complex than in the case of the 1:1 complex. The standard enthalpy and the standard entropy of formation of the 2:1 complex could be estimated : Delta H degrees = -16 +/- 5 kJ mol(-1) and Delta S degrees = -28 +/- 17 J K-1 mol(-1). It is speculated that this complex could be an intermediate in the pathway of the exchange of sodium cations between 1:1 complexes in solution.