The electron absorption coefficient (1 − δ̄) of an insulating surface held at an average potential V̄s between zero and the first crossover potential is observed to increase substantially when the insulator is subject to excitation from a separate electron beam or light source. This effect is apparently caused by the migration of holes to the vacuum interface under influence of the applied field. A vidicon gun in the low-velocity scanning mode was used to establish a potential V̄s of the exposed surfaces of thin films of As2S2 and As2S3 up to 5 μ in thickness which were evaporated onto electron-permeable or light-transparent conducting substrates. The average insulation excitation current ĪT was measured as a function of IB, the low velocity beam current, for fixed values of VT, the potential of the conducting substrate, under constant excitation conditions from either a light source or a 20 keV electron beam extended area source. The average electron absorptivity (1 − δ̄) of the insulator is determined as a function of V̄s, using the equation ĪT = (VT − V̄s)/R = (1 − δ̄)IB on the assumption that resistance R of the insulating film is independent of the field gradient applied across it, for a fixed value of electron bombardment (or light) excitation. Observations also include measurements on changes in (1 − δ̄) after evaporating thin layers of different, metal and insulating films over the exposed surfaces of the insulating films.