A detailed investigation of tunneling has been carried out on Nb-Nb2O5-Pb(In) junctions where the tunnel barrier is made-of Nb2O5thermal oxide with accurately controlled oxidation parameters. The oxide barrier appears as a n-type semiconductor with two Schottky barriers at the interfaces. The transport of charged oxygen vacancies is supposed to play a fundamental role in the oxidation process. The barrier potential φ and the tunnel resistance R have been related to the oxidation parameters. The junctions realized have sizes ranging from 4 to 104μm2, the Josephson current density, dependent on the oxide thickness, is in the 1-105A/cm2range. Very low transition times from the superconductive to the normal state have been observed.