Two-dimensional molecular-packing structure and monolayer preparation of adenine adsorbates on the graphite ͑0001͒ surface have been studied using scanning tunneling microscopy, low-energy electron diffraction, and thermal-desorption spectroscopy. By combining real-space images and diffraction data a close-packed hydrogen-bonded network of adenine dimers is proposed, containing two dimers in a unit cell with the symmetry group p2gg. The energy-minimized molecular arrangement could be determined by force field calculations. Adenine adsorbate layers were prepared by sublimation in UHV. ͓S0163-1829͑97͒02403-X͔