We have measured the magnetoresistance versus bias voltage (Vbias) in NiFe/TaOx/Al2O3/Co magnetic tunnel junctions. The TaOx layer was produced exposing a thin metallic Ta film (0.25 or 0.5 nm) to atmosphere before the deposition of Al2O3. The samples with 0.5 nm of Ta present a faster decrease of the magnetoresistance for increasing Vbias. They also present negative tunnel magnetoresistance values for Vbias larger than 250 mV. The results are discussed in terms of the oxidation of the Ta layers and may suggest quantum well state effects.