The fluorescence enhancement of 1,3-calix[4]bis-(10-cyano-9-anthrylmethyl)-o-benzo-crown-6 (AnCN2) and of 1,3-calix[4]-25,27-bis(allyloxy)-26,28-(10-cyano-9-anthrylmethyl)-o-benz o-crown-6 (AnCN1) upon complexation of Cs+ have been studied. The fluorescence intensity vs. [Cs+] profiles suggests the 1:1 stoichiometry for host AnCN1 with Cs+ but 1:2 stiochiometry for AnCN2 with Cs+. The 1:2 stiochiometry complexations of AnCN2 with Cs+ under low and high AnCN2 concentrations are also studied. A stepwise two cesium ions complexation occurs in the fluorescence intensity vs. [Cs+] profiles of AnCN2 at low concentrations (<10(-5) mol/l). But such stepwise complexation cannot be observed at higher ligand concentration. A high concentration leveling effect is proposed to explain this phenomenon. <(c)> 2001 Published by Elsevier Science B.V.