The accumulation and localization of the positive charge generated during Fowler-Nordheim electron injection in the buried oxide (BOX) of silicon implanted by oxygen (SIMOX) silicon-on-insulator structures have been studied. It is determined that the generated positive charge is localized near the BOX-silicon substrate interface, independent of the injected electron source (film or substrate). The positive charge generation probability in the SIMOX BOX is found to be considerably higher than that in thermal oxide. The generated positive charge is stable at thermal anneal temperatures up to 400 degrees C, but we demonstrate for the first time that an rf hydrogen plasma treatment is very effective in reducing this positive charge. The mechanism of rf plasma annealing of the positive charge in the BOX layer is discussed.