MIS capacitors have been developed on a GaAs (n) substrate, the insulation being a thin layer of polymethylsiloxane (a few thousand angstroms thick). The present aim is to find out whether this process can provide an answer for the passivation of compound‐semiconductor components. The thin layer is deposited on the gallium arsenide substrate. It is obtained by glow discharge in a hexamethyldisiloxane vapor at ambient temperature. TheC‐Vcurves enable the polymer‐semiconductor interface properties to be determined, and in particular the calculation of fixed charges contained in the polymer (QT) and the surface‐state density (Nss).