The intrinsic pK value (pK(i)) of the phosphate group of cardiolipin liposomes was found using an original method which resembles the electrochemical adsorption technique conventionally used in studies of the electrical double layer. Our method is based on the measurement of changes in proton concentration Delta C-H+ in the diffuse layer adjacent to the charged lipid membrane, as a result of stepwise changes of monovalent salt concentration in the bulk solution. PKi(PO4-)=2.5 (+/-0.1) was found by fitting the theoretical Delta C-H+ vs. logc curves to the corresponding experimental values in the range 10(-3)-10(-1) M sodium chloride. Changes in the membrane charge density at various pK(i) are presented. The advantages and limitations of the suggested method are also discussed.