The surface-specific process of optical second-harmonic generation has been applied to investigate adsorbed molecular monolayers exhibiting a periodic modulation across the surface. In addition to the usual reflected second-harmonic signals, these spatially modulated monolayers are found to give rise to several orders of diffracted second-harmonic radiation. An analysis is presented relating the characteristics of the second-harmonic diffraction pattern to the spatial properties of the modulated adlayer. In this study, gratings in the adsorbate density of dye molecules adsorbed on insulating substrates were formed by photo-desorption in the field of two interfering laser beams. From the second-harmonic diffraction data, adsorbate density profiles have been inferred. These results can be explained by a model for the formation of the molecular gratings based on a thermal desorption mechanism.