Diffracted surface second-harmonic radiation emerging in several orders has been observed from a periodically modulated monolayer of adsorbed dye molecules. The molecular grating was produced by laser-induced desorption in the field of two crossed beams. An elementary theory is presented that relates the characteristics of the second-harmonic diffraction pattern to the spatially varying properties of the surface and is applied to infer the adsorbate density profile of the spatially modulated grating. The density profile is compared with the predictions of a model of grating formation based on thermal desorption. © 1989 Optical Society of America.