MgO is an efficient emitter of exoelectrons. In this research, electron capture in Si/SiO2 substrate during thermally stimulated exoelectron emission (TSEE) measurements of MgO thin films was studied. Si/SiO2 is a substrate commonly used in microelectronics. The substrate consisted of an amorphous 1 µm thick SiO2 layer thermally grown on a Si wafer. Nanocrystalline MgO films were prepared on the substrate by the extraction-pyrolytic method. TSEE from the films was measured and strong TSEE peaks were observed at temperatures of 450 ºC and 525 ºC. Also, photoelectron emission (PE) from the films was measured before and immediately after TSEE measurements. PE was excited by UV photons of 4–6 eV energy and it was found that the registered photoelectrons were emitted both from the film and the substrate. PE spectra recorded after TSEE measurements had several distinct PE maxima that were associated not with MgO but with electrons trapped in the defect centres of the SiO2 layer. We attribute the origin of the defect centres to the SiO2 layer since the same PE maxima were observed in PE spectra of a bare Si/SiO2 substrate after its irradiation with weak electrons with energies up to 1.5 keV emitted by a hot cathode source. Presumably, the defect centres already existed in the as-fabricated SiO2 layer and they could capture exoelectrons emitted by MgO films. To check whether the substrate was charged during TSEE measurements of MgO films, the bare Si/SiO2 substrate was also subjected to TSEE measurements. PE spectra of the bare substrate recorded after its TSEE measurements did not demonstrate any PE maxima. Therefore, our findings suggest that the defect centers that already existed in the SiO2 layer were filled with electrons emitted from MgO film during its TSEE measurements.