The surface voltage of ultra-thin insulator (native oxide) layer upon silicon substrate has been measured by the vibrating capacitor (Kelvin) method. A change of some hundreds of millivolts in the surface voltage has been observed on the illuminated ultra-thin insulator-Si system. The original (dark) value of the surface voltage is reached through a transient process. The present article discusses the origin and nature of this surface voltage transient (charging and discharging of the surface) and evaluation of the measured results for calculation of the tunnel current through ultrathin insulator layer. (C) 2002 Elsevier Science Ltd. All rights reserved.