Abstract <p>The possibilities of return-path laser ellipsometry for monitoring oxidation–reduction processes on metals and alloys in rarefied environments have been studied. A laser automatic ellipsometer has been built based a return-path autocollimation scheme, which makes it possible to remote monitoring of the growth and degree of oxidation of the surface layer on metal samples through the optical window of the vacuum chamber. It has been shown that kinetic return-path ellipsometry makes it possible to record confidently the onset of a new phase formation based on changes in the slope of the experimental ellipsometric nomogram Δ(<i>t</i>)–ψ(<i>t</i>). As an example, laser-ellipsometric studies of low-temperature vacuum oxidation processes of AISI-321 alloy through the optical window of the vacuum chamber of an Auger electron spectrometer have been performed. A combination of kinetic return-path ellipsometry and Auger electron spectroscopy with layer-by-layer etching has been used to show that after the induction period of oxidation, a kink is recorded on the ellipsometric kinetic curve Δ(<i>t</i>)–ψ(<i>t</i>), which is associated with the onset of formation of a thin (40–50 Å) metal–oxide layer of a new oxide phase in the volume due to the exchange reaction of iron oxides with chromium from the alloy and the resulting conversion of iron oxides into spinel (FeCr₂O₄). In this case, the front of this transition shifts closer and closer to the metal–oxide interface, while the formation of low-protective iron oxides becomes increasingly difficult, and they do not have time to reach their maximal thickness.</p>