Electric capacity and electric conductance of lipid membranes composed of phosphatidylethanolamine and alpha-tocopherol were determined by electrochemical impedance spectroscopy. These components were chosen for the study because they are present in biological membranes and they fulfill essential functions in lively organisms. A domain structures appear as a result of phosphatidylethanolamine-alpha-tocopherol membrane formation. Formation of the domains can explain the deviation from the additivity rule. This equilibrium was described by mathematical equations and was further verified experimentally. Based on the derived equations, the surface area of domain was calculated; it amounts to 320 angstrom(2). This value is consistent, taking into consideration ordering and condensation effects of alpha-tocopherol on the phosphatidylethanolamine, with a stoichiometry of such a domain equal to 4:1. (C) 2008 Elsevier Ltd. All rights reserved.