Molecular magnetic tunnel junctions using self-assembled monolayers (SAMs) as tunnel barriers show stable and efficient spin transport properties. Large tunnel magnetoresistance with a flat bias voltage dependence of the magnetoresistance is observed in La2/3Sr1/3MnO3/dodecylphosphonic acid SAM/Co nanocontacts. This opens the door to spintronic tailoring though SAM engineering and could also lead to new venues for spin injection in organic devices.