The paper discusses the results of an experimental investigation concerned with the transverse magneto-resistance effect in very thin semiconducting films situated in a microwave field at 10Gc/s, with a superimposed steady magnetic field, of flux density up to 0.45 Wb/m2, applied at right angles to the surface of the film. The film used was of indium antimonide with a thickness of the order of 103Å, much less than the skin depth at the frequency employed. It was deposited by evaporation in a vacuum on a thin sheet of mica and placed longitudinally in a rectangular waveguide with the surface of the film parallel to the narrow wall of the guide, in much the same way as in the ordinary vane-type attenuator. The steady magnetic field, applied transversely to the film, produces a change in its conductivity and a corresponding effect on the microwave transmission properties. This magneto-resistance effect is related to the mobility of the carriers in the semiconductor, and consequently the technique enables this mobility to be determined without the use of electrodes attached to the specimen.