The effect of pH on the surface charge density of the O/W-type microemulsion formed in solutions of an amphoteric surfactant, (N-alpha, N-alpha-dimethyl-N-epsilon-lauroyllysine, DMLL)/n-octane/1-pentanol/CaCl2 or AlCl3, has been investigated. A novel equation for the surface charge densities in the presence of a multivalent cation (Ca2+ and/or Al3+) has been derived taking account of the number of DMLL molecules bound to a multivalent cation (q). By applying a numerical analysis of this equation, it is possible to determine the intrinsic binding constants (K) of the multivalent cation and the monovalent anion to the hydrophilic groups of DMLL and the adsorption density (N) of DMLL molecules on the oil/water interface. Furthermore, N has been found to show pH dependence. N has a maximum value around the isoelectric region (in particular, pH 5-7) of DMLL molecules in the presence of CaCl2, whereas N increases with increasing pH in the presence of AlCl3. The surfactant (DMLL) numbers per O/W-type microemulsion droplet, i.e., the aggregation numbers, are successfully obtained as a function of pH in the presence of CaCl2 and/or AlCl3.