The dielectric properties of mixed monolayers of per-(6-amino-2,3-di-O-hexyl) beta-CD hydrochloride (NH3-beta-CD-OC6) and 1,2 dipalmitoyl, 3-sn-phosphatidic acid (DPPA) have been assessed using surface potential measurements at constant area, From the comparison of these surface potential (Delta V) versus surface density (delta) relationships with those of surface pressure (pi) against surface density (delta) it was apparent that the increase in the NH3-beta-CD-OC6 content in mixed films gave rise to a gradual increase in the saturation value of the surface potential (Delta V-max). This potential for pure DPPA was found to be equal to 396 mV and for pure CD 554 mV. The Delta V-max values reflect the onset of reorientation effects that arrive at molar areas before the collapse of these films, Independently of reorientation effects, the obtained results strongly indicate that the dipolar term contributing to the overall Delta V value was for NH3-beta-CD-OC6 due to the hydration of its NH : group, For both DPPA and NH3-beta-CD-OC6 molecules the contribution of the electric double layer (psi) was calculated and was found for DPPA and NH3-beta-CD-OC6 to be equal to -249 and +252 mV, respectively, These calculated psi values made possible the evaluation of dipole moments for NH3-beta-CD-OCB and DPPA monolayers which revealed a marked difference in dipolar properties between these two film forming components, In contrast to DPPA which exhibited a decrease in the surface dipole moment (mu(perpendicular to)) with the decrease in A, NH3-beta-CD-OC6 displayed an increase in mu(perpendicular to) with the decrease in A for A values above 580 Angstrom(2). Below this value mu(perpendicular to), decreases with decreasing molecular area and this variation arises from a change in the polarity of the electric double layer arising from interactions with the complementary anion, The differences in dielectric properties between the two film forming molecules have been attributed to modification, during compression, in the structure of the interfacial water bound to the cyclodextrin.