The spectral dependence of positive space charge induced in an Al‐Al2O3‐Si MOS structure shows a sharp threshold at 7.8±0.2 eV. This threshold is interpreted to be the onset of band‐to‐band transitions in Al2O3. The net space charge induced by 10.2‐eV radiation can be positive or negative depending on the bias polarity and magnitude. The voltage dependence is qualitatively explained by the competitive processes of electron‐hole pair generation and electron injection from the electrodes.