Thin‐film sandwiches of Al&sngbnd;Al2O3&sngbnd;Al have been constructed with Al2O3layers in the range 80–120 Å thick. When illuminated with light in the wavelength range 2600–11 000 Å, the sandwiches exhibit photocurrents which are larger than the dark current. The photocurrent has been measured at room temperature as a function of bias voltage and photon energy, using light pulses of 0.1‐sec duration. The results are interpreted to give the shape of the potential barrier in the insulator. Within the accuracy of the data, the potential barrier is trapezoidal, with a relative work function of 1.0 eV at the oxidized electrode, and 2.9 eV at the electrode evaporated over the oxide. There is evidence that collisions of the photoelectrons in the insulator are important.