Partially deuterated fatty acid and phospholipid Langmuir films were examined by infrared reflection-absorption spectroscopy, thereby permitting a conformational analysis of different regions of the him-forming molecules from the frequencies of the CH2 and CD2 stretching vibrations. For hexadecanoic acid-d(17), half-deuterated at the tail end, the observed CH2-stretching frequencies of approximate to 2915 and 2849 cm(-1) were considerably lowered from those of completely protiated hexadecanoic acid, indicating that the conformational order adjacent to the headgroup is much higher than at the tail end. With the half-deuterated fatty acid as the acyl chain component in DPPC-d(34), it was possible to probe different parts of the hydrocarbon moiety of phospholipid monolayers at the air-water interface, Results similar to those for the fatty acids were noted, with greater conformational disorder occurring toward the bilayer center.