A detailed investigation of tunneling in Pb/PbO/Pb junctions has been carried out, in an attempt to independently measure several of the parameters known to enter the tunneling problem. In particular, the thickness of the tunnel barrier (orthorhombic PbO thermally grown in situ in an ultraclean vacuum system) has been measured ellipsometrically, and barrier heights of 1.15 ± 0.15 eV at both metal-oxide interfaces have been deduced from Fowler-Nordheim plots and logarithmic conductivity measurements. The junctions are of high quality (less than 0.1% nontunneling currents) and cover a Josephson current density range of from 10-4 to 103A/cm 2. From the dependence of the current density on the barrier thickness, an effective mass ratio, m*/m of 0.5 ± 0.1 has been deduced for electrons in the barrier. Comparison of experimental results with the tunneling theory shows that to obtain an agreement between them, image-force corrections must be neglected. Reasonable quantitative agreement has been obtained with a small correction of 4-5 Å to the ellipsometric thickness values in the range 19-31 Å; no other adjustments are found necessary. This discrepancy might be attributed to a difference between ellipsometric and tunneling thickness values. © 1974 American Institute of Physics.