Complex formations between calix[4]arene-bis(crown-6-ether) calix-COU2 (A1) and the tetrasulfonated species calix-COUSULF (A2) with Cs(+) are investigated in water and ethanol, and in 9:1 (M1) and 1:9 (M2) H(2)O/EtOH v:v mixtures, by chemical relaxation and molecular modeling. In ethanol and M2, two Cs(+) are included in A1 in two kinetic steps, whereas complex formation in M1 becomes controlled by a slow first-order kinetic process, which is accompanied by very fast Cs(+) inclusions, second-order rate constant: k'(1) = (3.4 +/- 0.8) x 10(7) M(-1) s(-1). In water and M1, A2 forms 1:1 and 1:2 cesium complexes in a single kinetic step, whereas in M2, two Cs(+) are included in two kinetic steps. The rate and thermodynamic constants involved are reported. They show that the second-order rate constants increase with the ethanol-to-water ratio, e.g., A2, second-order rate constant for the first Cs(+) in water: k(1A2water) = (9.7 +/- 0.3) x 10(4) M(-1) s(-1) and in M2: k(1A2M2) = (6.3 +/- 0.4) x 10(9) M(-1) s(-1). The affinities of both A1 and A2 for Cs(+) also increase with the ethanol-to-water ratio, e.g., first inclusion of A1 in M1: K(1A1M1) = (5 +/- 1.3) x 10(3) and in ethanol: K(1A1EtOH) = (7 +/- 3) x 10(6). The deviation from the expected mechanism of complex formation with alkali is attributed to the comparatively more difficult access of Cs(+) to the inclusion cavity of the capped calixarene. An analysis of calix-COU2 and calix-COUSULF and their Cs(+) complexes with only one rim capped by the crown ether confirms the thermodynamic and kinetic results, by showing that the inclusion cavity of calix-COUSULF is more adapted to Cs(+) than that of calix-COU2. This added to the presence of the shielding effect of the negative sulfonates can explain that the affinity of calix-COUSULF for Cs(+) is higher than that of calix-COU2. These results can be of interest in the search of an efficient Cs(+) decontaminant.