Purines and pyrimidines slowly self-assembled from solution on a crystalline gold surface to form a dense ordered monolayer. The adsorbed layers have been imaged with scanning tunneling microscopy and atomic force microscopy and analyzed with electron spectroscopy for chemical analysis and static secondary ion mass spectrometry. It was found that the adsorption process occurs in three stages : (1) the molecules adsorb first in random clusters, (2) then the clusters adsorb at the reconstruction sites of the Au(111) surface, and(3)finally a structured overlayer is assembled. The surface lattice dimensions of the assembled film correspond to those of the bulk lattice. The driving force for the self-assembly is discussed.