The interaction of oxygen with initially-clean lead surfaces was studied using Auger electron spectroscopy to determine the oxygen uptake and ellipsometric measurements to monitor the resultant oxide fila thickness. The results from both techniques were in very close agreement and indicated that a true oxidation process was operative over the pressure range investigated (5 × 10-8 to 5 × 10-6 Torr O2) at room temperature. The thickness of the growing oxide was closely proportional to the oxygen exposure (i.e., the pressure-time product) until a limiting value of 6.3 ± 0.2Å was reached, regardless of pressure. The refractive index of the film was 2.8 at gl = 5461Å, characteristic of the orthorhombic PbO crystal structure. The oxidation apparently proceeds via a place exchange mechanism wherein the activation energy barrier is markedly lowered by image force effects until a critical thickness is reached, dependent on the Pb-O single bond energy; from the dissociation energy of PbO and the atomic jump distance in orthorhombic PbO, this thickness was estimated as 6.5Å in good agreement with the experimental value. Analysis on the basis of chem-isorption did not yield satisfactory agreement. © 1973 American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.