We used scanning tunneling microscopy (STM) to investigate the local leakage current through ultrathin silicon dioxide(SiO2)films grown on Si substrates. Individual leakage sites, which were created by hot-electron injection from the STM tip under a high sample bias of+10 V,were identified from the local change in surface conductivity due to defect creation in the oxide films. When we reversed the stressing polarity (using a negative sample bias) no leakage sites were created in the oxide film. ©1998 American Institute of Physics.