The influence of vibrational motion on electron conduction through single molecules bound to metal electrodes is investigated employing first-principles electronic-structure calculations and projection-operator Green's function methods. Considering molecular junctions where a central phenyl ring is coupled via (alkane)thiol-bridges to gold electrodes, it is shown that - depending on the distance between the electronic it-system and the metal -electronic-vibrational coupling may result in pronounced vibrational substructures in the transmittance, a significantly reduced current as well as a quenching of negative differential resistance effects. (c) 2006 Elsevier B.V. All rights reserved.