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A new technique for growing SnO, thin films with high surface area is described, based on tin rheotaxial growth and its thermal oxidation (RGTO). Tin thin films, when grown, present a surface characterized by spheroidal agglomerates due essentially to the surface tension of the liquid metal; these agglomerates do not seem to have any electrical continuity. By means of a thermal oxidation, both the transformation of the metal into a semiconductor and the thin film continuity are obtained, owing to the volume increase during the above-mentioned phase transformation. After this annealing cycle, SnO, thin films are slightly oriented in the (101) direction and present an electrical resistivity equal to about 10' R cm. So, the surfaces of SnO, films appear to be formed by spongeous agglomerates, which are in electrical contact with the nearest agglomerates. SnO, thin films grown by this method show a high sensitivity (defined as the relative per cent conductance variation) to H, ; in fact, sensitivity to 200 PPM H, in synthetic air at ambient pressure is equal to 400 YO. It seems to be possible to prepare other metal oxide semiconducting thin films with a high surface area using this technique.