The crystalline structures of monolayers of the N-eicosanoyl-3-aminopropionic acid CH3-(CH2)18-CONH-(CH2) 2-CO2H and its mixtures with eicosanamide CH3-(CH2)18-CONH2 on a water subphase were determined, at near-atomic resolution, by grazing incidence X-ray diffraction (GID), using synchrotron radiation. The N-eicosanoyl-3-aminopropionic acid molecule must adopt a very particular conformation in order to ensure satisfactory packing requirements. The packing characteristics were tested by fitting the measured GID data using atomic coordinate models for various conformers, complemented by lattice energy calculations. The structure of the mixed monolayer in 1:1 and 1:2 molar ratios is different from that of each pure component. We propose that the two components do not form a random solid solution but rather the N-eicosanoyl-3-aminopropionic acid forms domains within the sea of eicosanamide molecules, stabilized by the interactions between the -(CH2)2-CO2H moieties. © 1993 American Chemical Society.