The surface pressure-area (pi-A) isotherms of enantiomeric polylactides and their equimolar blend reveal differences in the appearance of features and compression rate effects. Surface potential data indicate that the poly(L-lactide) monolayer interacts differently than the 50:50 poly(L-lactide)/poly(D-lactide) blend film with the water subphase. In the case of the 50:50 blend at 25degreesC, compression-expansion experiments confirm that the change in the monolayer is irreversible within the time frame of the experiment. In the poly(L-lactide) case, hysteresis is also observed, but the film readily reverts to its original state. Infrared spectra of the polymer monolayers at the air-water interface were obtained using the polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) technique. PM-IRRAS spectra of a compressed poly(L-lactide) monolayer are consistent with the polylactide helices lying in the plane of the air-water interface. No absorbances are observed in the PM-IRRAS spectra Of poly(L-lactide) prior to the plateau in the pi-A isotherms, suggesting that the helix formation is linked to the increase in surface density which accompanies film compression. The PM-IRRAS spectra establish that the conformation of the poly(L-lactide) helices with respect to the air-water interface is different than the conformation of the D/L blend helices.