A monolayer of 4-(octadecyloxy)benzoic acid at the air-solution interface has been used to induce oriented nucleation of three-dimensional (3-D) crystals of 4-hydroxybenzoic acid (HBA) monohydrate. The two-dimensional (2-D) crystalline structure of this monolayer in a self-assembled uncompressed state, both on a water subphase and on a solution containing HBA, was determined by grazing incidence X-ray diffraction (GID) using synchrotron radiation. Crystalline domains of large coherence length were formed on water subphase at 14 degrees C, and the fraction of the monolayer in the ordered state was much increased on HBA solution. The agreement between the observed and calculated GID data was found to be sensitive to the conformation and orientation of the monolayer molecules. These results demonstrate the ordered binding of the HBA solute to the amphiphile benzoic acid head groups to form hydrogen-bonded cyclic dimers between the two carboxylic acid moieties. The GID method was complemented by other surface sensitive techniques such as specular X-ray reflectivity, surface pressure-area isotherms, and nonlinear optics. A resulting model arrangement of the monolayer, based on the molecular packing, has been proposed for the induced nucleation of the 3-D crystals of 4-hydroxybenzoic acid monohydrate attached to the monolayer.