We present some observations of electron-filling modulation reflectance in charged self-assembled InxGa1 - xAs quantum dots. This electron-filling modulation reflectance is a different type of electroreflectance, which is based on the Pauli blocking of interband transitions in quantum dots. By adjusting the appropriate ac and de reverse biases, electron filling in the quantum dots can be modulated. Experimentally determined interband transitions have been compared with those obtained from photoluminescence spectra. The good agreement between these results reveals that at least three quantum-confined electron states are contained in our quantum dots due to their electron-filling character. As the temperature is increased, the relative intensity of each state can directly reflect the electron populations of the quantum states. The technique developed here provides an efficient way to observe the interband transitions of quantum dots. [S0163-1829(99)51028-X].