Dielectric breakdown is the process of local materials transiting from insulating to conductive when the dielectric is submerged in a high external electric field environment. We show that the atomistic changes of the chemical bonding in a nanoscale breakdown path are extensive and irreversible. Oxygen atoms in dielectric SiO2 are washed out with substoichiometric silicon oxide (SiO?? with ??<2)formation, and local energy gap lowering with intermediate bonding state of silicon atoms (Si1+, Si2+, and Si3+)in the percolation leakage path. Oxygen deficiency within the breakdown path is estimated to be as high as 50 percent -60 percent . We thank G. Zhang and V. L. Lo for technical discussions and sample preparation, and Chartered Semiconductor Manufacturing for providing the samples. This work was supported by Ministry of Education (MOE)Grant Nos. T206B1205 and NTU RGM 33/03.