STM was employed to study the coadsorption of 4.4'-bipyridine (4bipy) and 2-mercaptopyrimidine (2MPy) on Au(1 1 1). The adsorption of 4bipy as a single layer was also analyzed. For the single layer STM reveals a molecular structure made of parallel periodic chains with a repeat distance of 11.5-11.7 Angstrom. The mixed layer is formed with a ratio of two molecules of 2Mpy to one of 4bipy. 4bipy assumes two different positions in relation to the 2MPy dimer. Competition between 4bipy, 2,2'-bipyridine and 1,10'-phenanthroline was investigated to obtain a qualitative view of the preference of the: 2MPy assembly for these molecules. The mixed layers were grown on Au(1 1 1) from ternary molecular solutions containing either 2MPy + 4bipy + 2bipy or 2MPy + 4bipy + phen. STM shows that the resulting layers are made of the 2MPy dimeric assembly plus one of the oligopyridines. A split of the 2MPy dimer or the formation of mixed domains with the: three molecules present in the stock solution was not found.