On a metal-molecule-metal nanojunction, the scanning tunneling microscope (STM) scans at the precise location of the electronic metal-molecule interaction permit a measurement of the contact conductance G(0). The conversion curve between the change in the STM contrast Delta h due to this interaction and G(0) is presented for a series of conjugated molecular wires. At chemisorption distances, the two-valued character of the G(0)(Delta h) function is discussed, indicating experimental ways to evaluate G(0) as a function of Delta h for different metal-molecule interaction ranges. (c) 2005 Elsevier B.V. All rights reserved.