A simple non-destructive method for determining the concentration of vacant deep traps in the vicinity of the film-substrate interface under extrinsic illumination is proposed and tested. The proposed technique relies on capacitance-voltage measurements under illumination. An increase in the width of the conductive channel under extrinsic illumination is expressed in terms of the structural parameters and the illumination-induced shift of the inflection point in the capacitance-voltage curve. The experimental results for GaAs epitaxial films are in satisfactory agreement with those obtained by an independent method based on sidegating measurements. (C) 2003 Published by Elsevier Science Ltd.