At the exact contacting distance of a CO molecule inserted in the tunnel junction between a Cu(3 1 1) surface and an STM tip, it is demonstrated that the calculated conductance of a single CO is smaller (G = 6 x 10(-4) G(o) with G(o) = 2e(2)/h) than the corresponding vacuum junction conductance. This is not due to a destructive interference between tunnel channels through the junction but owing to the very large CO-Cu(3 1 1) electronic coupling which desequilibrates the electronic structure of the tunnel junction. A molecule chemisorbed on at least one of its contacting electrodes is not a guaranty for a large junction conductance. (C) 2009 Elsevier B. V. All rights reserved.