The miscibility of per-(6-amino-2,3-di-O-hexyl) beta-CD hydrochloride salt abbreviated NH3-(beta)-CD-OC6 with 1,2 dipalmitoyl, 3-sn-phosphatidyl choline (DPPC) and 1,2 dipalmitoyl, 3-sn-phosphatidic acid (DPPA) and of per-(6-dodecanoylamino-6-deoxy) beta-CD abbreviated C11CONH-beta-CD with DPPC has been assessed by surface pressure-area experiments under dynamic conditions. The determination of their mixing behavior has been approached by thermodynamic characterization derived from a comparison of the composition dependence of collapse pressure, for various contents of two components in monolayers, with that predicted from a model of ideal mixing. Component miscibility has additionally been addressed by the use of the Smaby-Brockman state equation for liquid-expanded monolayers which characterizes interaction between monolayer components using the water activity coefficient, While for both NH3-beta-CD-OC6-DPPC and NH3-beta CD-OC6-DPPA systems the average surface areas followed the additivity rule, for the latter system surface pressures at collapse significantly deviated from ideality, The occurrence of an interaction at the level of oppositely charged polar groups of NH3-beta-CDOC6 and DPPA was also demonstrated by the nonideal composition dependence of the water activity coefficient (f(1)) characterizing the interfacial water. The mixing energies and interaction parameters at collapse pressures were calculated using the Joos approach. Positive values of these factors indicated that NH3-beta-CD-OC6 and DPPA mixed films were thermodynamically less stable than the films in which ideal mixing of components occurred. The mixing of NH3-beta-CD-OC6 with DPPC appeared to be almost ideal, In the case of the C11CONH-beta-CD-DPPC system, analysis according to both the Joos and Goodrich approaches showed the occurrence of an important interaction which resulted in negative mixing energies characteristic of thermodynamically stable mixed films.