Previously proposed models of monolayers of adenine and guanine, based on scanning tunneling microscopy and atomic force microscopy images, have been critically examined. We have applied molecular mechanics computer simulations to these models and, where appropriate, examined additional scanning tunneling microscopy data. These findings support the proposed adenine structure based on low-energy diffraction analysis and molecular mechanics but indicate that the structure of the guanine monolayer on graphite is different from those previously proposed. These findings indicate that the energy-minimized adsorbate stuctures of both adenine and guanine form a similar molecular configuration on both graphite and molybdenum disulfide surfaces. It is speculated that the formation of these structures may have had some prebiotic relevance.