An amino acid surfactant having an optically active base as a counterion was synthesized by neutralization of N-acyl amino acid with 1-phenylethylamine (PEA). Phase diagrams of diastereomeric mixtures of R-PEA N-acyl DL-amino acid and RS-PEA N-acyl D-amino acid systems were obtained by differential scanning calorimetry. The functions of N-acyl amino acids and PEA as optical resolving agents were examined by drawing the phase diagrams. FT-IR studies suggested that the difference in the magnitude of the interaction between the amide groups was a dominant factor affecting the difference in the physicochemical properties between the diastereomers.