A novel Langmuir monolayer based an 1-octadecanesulfonate amphiphiles spread over an aqueous subphase containing guanidinium cations is described. The molecular area of this monolayer deduced from pressure-area isotherms agrees with that calculated for two-dimensional pseudohexagonal hydrogen-bonded networks found in numerous guanidinium alkane- and arenesulfonates. The compressibility behavior of the monolayer suggests puckering of the hydrogen-bonded network due to the applied pressure and/or the existence of stable domains of two-dimensional crystallites which coalesce on compression. When benzenesulfonic acid is present in the subphase, these monolayers induce the oriented nucleation of crystals of diphenyl sulfone, the decomposition product of benzenesulfonic acid. The observation of oriented nucleation and growth in the absence of a specific stereochemical match between the monolayer and the (100) face of diphenyl sulfone suggests that the nucleation process may involve coincident epitaxy between the two-dimensional monolayer and the (100) face, which selves to maximize dispersive interactions at the interface and lower the free energy of incipient nuclei.