A gas-discharge counter with a cathode of semitransparent aluminized mylar film is found to be sensitive to visible light. Time spectrum of counter signals caused by a pulsed light source showed that the sensitivity to light is due to cathode photoeffect. The proposed mechanism for this phenomenon is based on a decrease in the electronic work function of the aluminium cathode by the electric field in conformity with the Schottky-law. Positive ions arising in the gas discharge are deposited on the dielectric oxide film on the aluminium cathode surface. The resulting surface charge produces a strong electric field at the cathode ( Eσ ∼ 10 7V/cm) which decreases the work function and shifts the red boundary of photoeffect on the aluminium from the UV to visible region of the spectrum. The effect of this phenomenon on the operation of gas-discharge detectors and its possible applications are discussed.