Thin films of MoO 3 for gas sensing applications have been deposited by reactive sputtering using a metallic molybdenum target. After annealing at 500°C in air, the layers consist of rhombic molybdenum trioxide with a (010) texture. The morphology of the layers considerably depends on the film thickness. Electrical conductivity measurements with different test gases show that MoO 3 layers are suitable for NH 3 detection at operating temperatures between 400 and 450°C.