Rectification of electrical current was observed in a Langmuir-Schaefer monolayer of fullerene-bis[ethylthio-tetrakis(3,4-dibutyl-2-thiophene-5-ethenyl)-5-br omo-3,4-dibutyl-2-thiophene] malonate, Au electrodes at room temperature (there are two regimes of asymmetry, at lower bias, i.e., between 0 and +/- 2 V, and at higher bias), and also between Pb and Al electrodes at 4.2 K. The latter experiment was coupled with second harmonic detection of the second derivative of the current with respect to voltage (d(2)I/dV(2)). The d(2)I/dV(2) spectrum shows intramolecular vibrations, and also two antisymmetric broad bands, centered at +/-0.65 V, due to resonant electron tunneling between the Fermi level(s) of the electrodes and the lowest unoccupied molecular orbital of the molecule.