Cesium-137 is considered as one of the most dangerous radioactive materials that can be very easy move into the environment and contaminate it. This study aims to determine the optimum conditions for cesium/ sodium separation by nanofiltration (NF)-complexation on Nanomax 50 membrane with resorcinarene as the complexing agent. Protonation constants of resorcinarene were determined by potentiometric titration in 0.1 mol/L NaNO3 at 25degreesC (log K-H1 = 9.24 +/- 0.06; log K-H2 = 210.21 +/- 0.03; log K-H3 = 8.59 +/- 0.07; log K-H4 = 7.85 +/- 0.08). The apparent formation constants of the Cs+-resorcinarene (log K-CsL) were determined by UV spectrophotometry to evaluate the selectivity of resorcinarene toward cesium complex formation as a function of the pH. Parameters such as the pH, [resorcinarene]/[Cs+] ratio were studied to reach 80% of cesium rejection in 3 mol/L of NaNO3. The stability of resorcinarene was studied as a function of the pH and the concentration of NaNO3 by H-1 and C-13 NMR studies in D2O. Resorcinarene stability up to 30 days in [NaNO3] = 2 mol/L at pH 9 was determined, although at pH 12 and with the same sodium concentration, resorcinarene stability was 5 days. Finally, the conditions to reach 80% of cesium rejection in the presence of 2 x 10(4) fold of sodium were found at pH 10 with [resorcinarene]/[Cs+] ratio of 25 or at pH 11 with [resorcinarene]/[Cs+] ratio of 20.