Self-assembled monolayers of alkanethiols have been formed on oxidized surfaces of polycrystalline copper. For comparison, the monolayers were formed on clean copper surfaces. X-ray photoelectron spectroscopy (XPS), contact angle analysis, ellipsometry, and gas chromatography-mass spectrometry (GC-MS) have been employed to investigate the structure, formation, and thermal behavior of these monolayers. The results indicate that the structures of the monolayers on both surfaces are similar in quality, despite the fact that the oxidized copper surfaces were covered with the oxide layers of similar to 500 Angstrom thickness before monolayer formation. However, the monolayers on the oxidized copper are less stable than those on the clean copper against thermal heating in air. The observation is discussed in terms of the desorption mechanism of the monolayers. XPS and GC-MS analyses for the monolayer formation on the oxidized copper surface show that thiols are actually changed to disulfides with reduction of the oxide layers.