We have studied the electronic properties of conjugated and saturated dithiol molecules sandwiched between two Au(111) electrodes using first principles density functional calculations with a slab geometry. Relaxation of the molecule/surface adsorption geometry as well as the extended character of the metal electrode states are fully taken into account by our approach. Investigated quantities include the alignment of molecular energy levels with the Fermi energy (E-F) of the metal, the charge transfer and electrostatic potential profile, and the local density of electronic states (LDOS) at E-F. The behavior of the LDOS for benzene-, dibenzene-, and xylyl-dithiol molecules is analyzed and compared with that of alkane-dithiols of various lengths. (C) 2003 American Institute of Physics.