The intrinsic K-a values of the phosphate group of phosphatidic acid (PA) in self-organized monolayers deposited on a hanging mercury drop electrode were determined by a novel procedure based on measurements of the differential capacity C of this lipid-coated electrode. In line with the Gouy-Chapman theory, plots of 1/C at constant bulk pH and variable KCl concentration against the reciprocal of the calculated diffuse-layer capacity C-d,C-0 at zero charge exibit slopes that decrease from an almost unit value to zero as the absolute value of the charge density on the lipid increases from zero to approximate to 2 mu C cm(-2). The values so determined are K-1 = 10(8) M(-1) and K-2 = 10(4) M(-1). The plots of 1/C against 1/C-d,C-0 for PA exhibit slopes that pass from zero to a maximum value and then again to zero as pH is varied from 7.5 to 1.5, indicating that the charge density of the lipid film passes from slightly negative to slightly positive values over this pH range. An explanation for this anomalous behavior is provided.