The creation of clean Li surfaces by vapor deposition and their subsequent interaction with oxygen have been studied by a combination of ellipsometry and Auger electron spectroscopy in ultrahigh vacuum. The ellipsometric parameters Delta and Psi for Li thin films vapor-deposited on a Ni substrate were monotonic functions of the film thickness, and in reasonable agreement with theoretical values calculated for dense (nonporous) films. The clean Li surface is very reactive sticking coefficient of unity at the initial stage of oxidation at 5 x 10(-8) Torr O-2. Oxidation of the entire film (75 Angstrom maximum) into Li2O proceeded with an approximately unit reaction probability. A model of oxidation with in-plane contraction of the oxide film (continuously exposing fresh Li) is proposed to explain both the high rate of reaction and the observed variation of Delta and Psi with time of exposure to O-2.