The polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) technique has been used in situ to determine the orientation and molecular structure of an equimolar mixture of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) spread at the air-water interface. The characteristics of the compression isotherm and of the PM-IRRAS spectra give clear evidence for the presence of a PLLA/PDLA stereocomplex. One of the most striking features in the PM-IRRAS spectra of the stereocomplex is the derivative shape of the band due to the C=O stretching vibration, providing a spectral signature of the presence of polylactide helices oriented parallel to the water surface. The positive and the negative components of the C=O band observed at 1749 and 1765 cm(-1) are assigned to the A and E modes of the helical structure, respectively. This assigment was confirmed by recording transmission spectra of the transferred stereocomplex at normal and oblique incidence. Compression of the monolayer past 17 Angstrom (2)/repeat unit results in the formation of a bilayer structure. The surface pressure-area isotherm and the PM-IRRAS features suggest that the structure of the film at the air-water interface is similar to the three-dimensional crystal structure of the PLLA/PDLA stereocomplex. In the bulk crystalline structure, the molecules adopt a 3(1)-helix conformation, and a segment of a PLLA molecule is paired with a segment of a PDLA molecule, resulting in a racemic unit cell. The PM-IRRAS technique is thus shown to provide detailed insight into the structure of these polymeric Langmuir films and definitely shows that helical polymeric structures can be directly observed at the air-water interface.