The electrochemical properties of clean aluminum in LiClO4(PEO) solutions have been investigated in ultrahigh vacuum using as electrodes both foils and thin films vapor deposited on boron-doped diamond (BDD) layers supported on Si substrates. Voltammetric scans recorded at temperatures of about 55 degrees C yielded a set of deposition/stripping peaks at potentials positive to the onset of Li/Al alloy formation, attributed to Li underpotential deposition on Al. The positions of these peaks were found to be in relatively good agreement with those predicted by Gerischer-Kolb's work function correlation, as well as with recent theoretical calculations reported by Lehnert and Schmickler. In the case of Al films supported on BDD/Si, the charge under the stripping peak obtained after polarizing the Al electrode at potentials slightly positive to Li bulk deposition was found to increase with the film thickness. Further optimization of this type of experiments, by using, for example, a quartz crystal microbalance to determine the actual amount of Al deposited on the BDD/Si substrate, will make it possible to measure the stoichiometry of the alloy as a function of the applied potential.