We report on the use of scanning tunneling microscopy-conductance mapping to image voids in a thermally decomposed wet-chemical silicon oxide. Prominent contrast is observed between regions of remaining oxide and atomically clean silicon surface regions due to the difference in the local density of electronically active surface states. Differences in measured tunneling spectra within the voids, and in the surrounding oxidized regions, confirm that the origin of the contrast is mainly due to surface Fermi-level pinning at the clean Si(100) surface and metal-insulator-semiconductor junction behavior at the oxidized region. The maps show little sensitivity to pure topographical features, such as steps, and allow selective probing of electronic variations across a device structure with a resolution of 2 nm or better. (C) 2003 American Vacuum Society.