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An inversion of spin polarization has been observed in spin-dependent tunneling (SDT) junctions with Ta 2 O 5 and Ta 2 O 5/Al 2 O 3 barriers. The resistance of an SDT junction is found to be lower with magnetization of the ferromagnetic electrodes aligned antiparallel under specific voltage configurations. The tunneling magnetoresistance effect changes sign with applied voltage and varies from+ 1% to− 4% at room temperature. This inversion is believed to be due to the change in sign with bias of the spin polarization of one of the two electrodes. The strong dependence on voltage suggests negative spin polarization could arise from the densities of states for spins being different at the two electrode/barrier interfaces.