The influence of the density of metallic atoms on the formation of supramolecular structures onto Si surface alloys is investigated by scanning tunneling microscopy. Terephthalic acid (TPA) adsorption experiments are performed on the Si(111) alpha- and beta-(root 3 x root 3)-Bi surfaces. Although both surfaces have the same unit cell with Bi atoms incorporated into the Si substrate, they show a completely different behavior in respect to the formation of the supramolecular arrays. TPA molecules do not build any regular structure and adsorb randomly on the a surface but self-assemble into ordered and extended layers on the beta surface. This demonstrates that the absence of dangling bonds in metal-Si surface alloys is not a sufficient condition for supramolecular self-assembly but that the surface density of metallic atoms is a further essential parameter. (C) 2011 Elsevier B.V. All rights reserved.