The chiral discrimination of N-octadecanoylserine methyl ester monolayers at the air/water interface is investigated by external infrared reflection-absorption spectroscopy. For 297.15 K, evidence is presented that the alkyl chain of the racemic surface film in the liquid-expanded phase and in the liquid-expanded/ liquid-condensed transition region is less ordered than the enantiomeric one. In the condensed phase these differences disappear largely. At 293.15 K, the differences in alkyl chain order between the racemic and the enantiomeric films are reduced for large areas per molecule in comparison with the higher temperature, but they remain significant in the compressed state. In the case of the racemic film, the amide I band at 1626.0 cm(-1) is broadened in comparison with the enantiomeric film. The ester carbonyl group is slightly less protonated in the case of the enantiomeric film. The alcoholic C-O stretching vibration is found at approximate to 1083 cm(-1) and is more close to a horizontal orientation than to a vertical direction. In the case of the racemic film this band is broadened or split into two components at 1095 and 1079 cm(-1).