Electronic conductivity, mixed conductivity and Schottky-barrier sensors based upon the oxides SnOz and TiOz which are modified or contacted by the metals Pd and Pt can be used to detect gases like CO, CH4, H2 and 0,. The response signals of these different types of sensors result from changes in surface and bulk conductivities or in Schottkybarrier heights, which are measured using different geometric arrangements of metal (Pd or Pt) contacts. The atomistic understanding of sensing mechanisms and corresponding sensor structures is deduced from comparative spectroscopic and electrical measurements. Our studies show that reliable sensor properties can only be achieved either by forming stable metal/oxide interfaces or by adjusting a stable dopant distribution. layers evaporated onto Pt interdigital structures [3]. Two-point d.c. and a.c. conductivities were measured parallel to the surface as a function of the temperature and of the partial pressures of oxidizing and reducing gases such as 02, N02, CO, CH4 and H2. (2) Mixed conductivity (type 2) sensors consist of undoped Ti02 thin layers and twoand four-point arrangements of Pt contacts to determine d.c. and a.c. conductivities as a function of O2 partial pressure at high temperatures [4].