Thin interdigitated films composed of a long-chain, water-insoluble chiral acid (p-pentadecylmandelic acid of absolute configuration R) and a water-soluble chiral base (phenylethylamine, R′) were constructed at the air-solution interface. The (R, R′) structure was characterized to near-atomic resolution by grazing-incidence x-ray diffraction (GIXD). The two diastereomeric systems, (R, R′) and (R, S′), demonstrate similar surface pressure-molecular area isotherms, but their structures are completely different on the molecular level, as monitored by GIXD. Complementary data on these two architectures were provided by atomic force microscopy.