The properties of amphiphilic organic monolayers ("Langmuir films") at the air-water interface are strongly affected by metal ions in the aqueous subphase, and such monolayers can be used as templates to grow oriented crystals from dissolved salts. Our synchrotron X-ray studies of fatty acid monolayers spread on a dilute aqueous solution of lead ions indicate that a similar to5 Angstrom thick ordered layer is formed in the subphase. The lattice is commensurate with the organic lattice and has 14 times the unit cell area. It is unlikely that lead ions alone would form such a large repeat unit; indeed, using anomalous X-ray scattering, we detect no evidence of lead within the ordered layer. Thus, the interfacial superlattice is not simply an array of lead ions but may consist of lead hydrolysis products and water molecules.