The effect of DL-homocysteic acid (DL-2-amino-4-sulfonobutyric acid) on W/O microemulsion of potassium naphthenate (80%) and naphthenic acid (20%) in mixed solvent (1-octanol and n-heptane) has been found in four phases: (1) Interaction between the amino acid molecules and the polar head groups of the surfactant through hydrogen bonding enhances solubilization in the aqueous cores. (2) The interaction results in the growth of the microemulsion droplets and the homogenization of the particle size distribution. (3) The microstructure of the solubilized water remains unchanged, except that the polarity of the interface is affected. (4) The transition point is reduced to lower water content. A possible mechanism is proposed. (C) 2004 Elsevier Inc. All rights reserved.