The growth of ∼ 10 to 100 Å thick, orthorhombic PbO layers on lead was investigated by ellipsometry, electron tunneling and X-ray diffraction. Oxides were grown on 2000 to 15000 Å thick, vacuum-deposited lead films, using: oxygen pressures from 10-2 to 10+3 Torr; temperatures from -90 to + 150°C; and, times up to several weeks. Apparatus is described for monitoring ellipsometric parameters during oxidation and fabricating PbPbOPb tunnel junction devices in situ. The results indicate that lead recrystallization processes can accelerate the oxidation, so that the rates depend complexly on temperature, lead thickness and microsctructure. The refractive indices provide an indication of the extent of lead recrystallization effects on the oxidation process as well as the accuracy of the oxide thickness determination. X-ray findings indicate that the PbO has the orthorhombic PbO structure, is highly strained while being epitaxially related to the lead, and has a thickness close to that obtained ellipsometrically. Electron tunneling data provide a measure of oxide thickness fluctuations from the average or ellipsometrically-derived values. © 1973.