We demonstrate "on the fly" electron spin resonance ͑ESR͒ in which the defect generation process in the negative bias temperature instability ͑NBTI͒ can be observed without recovery contamination. Elevated temperature and modest negative gate bias generates ESR spectra due to EЈ center defects. The NBTI generated EЈ center spectrum disappears upon stress condition removal, a result consistent with recovery. Our observations support the idea that NBTI is triggered by inversion layer hole capture at an EЈ precursor site which leads to depassivation of nearby interface trap precursors.