Hot metal wires have been used in fouling tests, mainly when corrosion or chemical reaction is present. Most authors who have reported data obtained with hot wire techniques make no refence about the phenomenon related to the increase In surface area due to the accumulation of fouling layers. Yet this can cause problems of reverse insulation (critical radius), affecting the conclusions taken directly from the readouts. The aim of this paper is to discuss this problem and show through a few selected tases the Importance of this aspect in fouling resistance determinations using hot wire techniques.