Specimens of n-type galena were sulphurized, either by a wet chemical process or by exposure to sulphur vapour at various temperatures, and the resulting changes in rectification characteristics were observed, both with dc instruments and at 3 000 Mc/s, using various cats-whiskers. According to treatment given, either an improved n-type rectification was obtained or the polarity was changed to p-type The results are briefly discussed in the light of present-day rectification theory.