Experimental energy loss structures approximated by components at 11.3, 15.3, 18.3, and 23.2 eV for MgO and those at 14.5, 25.2, 35.3, and 49.9 eV for alpha-Al2O3 were compared with theoretical electron energy loss functions calculated from first principles using the full-potential linearized augmented plane wave method in the local density approximation. The electron energy loss functions, derived from the momentum matrix elements between Bloch functions, revealed that the experimental peaks at 23.2 eV for MgO and that at 25.2 eV for alpha-Al2O3 are due to bulk plasmon loss and the peak at 49.9 eV for alpha-Al2O3 is ascribed to the double losses of the plasmon excitation. The peaks at 11.3, 15.3, and 18.3 eV for MgO and those at 14.5 and 35.3 eV for alpha-Al2O3 result from the interband transitions from the valence band to the conduction band.