Surface pressure-area isotherms (Pi-A) of racemic and enantioenriched methyl 17,18-dihydroxy-octadecanoate (rac- and (17R)-17,18-DHO) at the air-water interface are very similar, while these eaters show significant differences in infrared reflection-absorption spectroscopy (IRRAS). Substantial shifts of the nu(as)(CH2) from 2927 to 2917 cm(-1) and of the nu(C=O) bands from 1722 to 1742 cm(-1) are observed upon monolayer compression from 0.8 to 0.2 nm(2) per molecule. This shift occurs gradually for rac-17,18-DHO and sharply within a small compression interval for (17R)-17,18-DHO. This suggests first, a change from a weak heterochiral to a weak homochiral preference, and second a removal of the two hydroxyl groups from the air-water interface, while the ester function immerses into the uppermost water layer in the compressed monolayer. This arrangement appears to be stabilized by intermolecular hydrogen bonds.