The electrostatic potential mapped on the electron density contour of gas phase molecules is used to identify several molecules physisorbed on transition metals imaged by scanning tunnelling microscopy within the molecular HOMO-LUMO gap. It is rationalized that the inverted of this potential, representing the potential energy felt by a negative test charge, is a good concept to interpret changes in the dielectricity constant through the influence of physisorbed molecules and thus to understand their submolecular resolution images. The validity of the concept is demonstrated on dichlorobenzene, chloronitrobenzene, and two azobenzene derivates on the surfaces of Au(111), Ag(111), and Cu(111). (c) 2007 Elsevier B.V. All rights reserved.