The interaction, at the air-water interface, between heptakis-6-O-(dimethyl-tert-butylsilyl)and calcium chloride has been studied using Langmuir isotherms and surface potential measurements. The results show that at CaCl2 concentrations below 5 x 10(-3) M there is coordination of the Ca2+ cations with bridging of cyclodextrin units leading to a more compact and stable monolayer and a decrease in apparent molecular area from 278 to 257 Angstrom(2). For Ca2+ concentrations between 5 x 10(-3) and 5 x 10(-2) M there is further coordination at the macrocyclic rim with inclusion of the anion accompanied by monolayer expansion and a decrease in monolayer stability. For CaCl2 concentrations above 5 x 10(-1) M there is inclusion of the cation with expulsion of anions and water from the macrocyclic cavity as shown by large changes in the surface potential and by the lack of change in apparent molecular area.