This paper reviews the current work carried out on mixed-oxide systems such as SnO2-WO3, TiO2-WO3 and SnO2-TiO2 with the ultimate aim of the application in gas sensing devices. Special emphasis is put on a relatively new system, SnO2-TiO2. Materials have been prepared in a form of polycrystalline ceramics and thin films by a variety of methods including impregnation, co-precipitation, sol-gel and sputtering. The effect of the composition on the microstructure and surface morphology is discussed. The importance of non-destructive optical methods of measurements, especially those based on the use of an integrating sphere, is pointed out. The sensor responses to hydrogen and methane are presented for SnO2-TiO2.