Thermally deposited monolayers of the DNA base guanine have been studied by scanning tunneling microscopy (STM) and a novel scanning thermopower microscope (STPM). The STPM signal provides a distinct representation of the monolayer lattice not present in the STM signal. The intensity of the thermopower contrast is dependent on the temperature gradient at the tunnel junction. Surface roughness and topographical variations of the underlying substrate are not convoluted with the signal due to the feed-forward acquisition method of this technique. A plausible interpretation of the STPM signal and images is proposed, and the prospects for single molecule differentiation and spectroscopy are discussed.