MgO films (2 nm thick) were grown on Mo and W substrates while metastable impact electron (MIES) and ultraviolet photoelectron spectroscopy (UPS) (HeI) spectra were collected in situ. Apart from the valence band emission no additional spectral features have been detected with electron spectroscopies. After exposing the oxide surface to Mg (substrate temperature between 100 and 300 K) an additional peak not seen with UPS, located within the band gap, shows up in MIES. This band, located at similar to 2 eV above the top of the valence band with a full width at half maximum of similar to 1 eV at the lowest exposures, can be detected in MIES until its intensity falls below a level of 10(-3) of that from the valence band. This additional emission is attributed to the formation of small, nonmetallic Mg clusters. The energetic position of the cluster emission closely matches that of the expected ionization of surface F-S(+)/F-S centers. It is proposed that in the initial phase of the Mg exposure, F center-type defects are produced close to extended defects, such as steps, corners, etc. These Mg-induced defects appear to play an important role as nucleation sites for cluster formation. At sufficiently large exposures a densely packed Mg film forms.