Scanning force microscopy is used to study the structure of various sensing films after chemical and thermal treatments. An example chemical modification of the sensing film can be achieved by heating a Pt/Ti sensing film in an oxygen environment. 1 With Pt acting as a catalyst, the Ti can become partially oxidized, resulting in a Pt/TiOx film which has different electronic properties than a metallic film. By further oxidizing the film, an insulating layer of TiO2 may be achieved. Or as in the case of hydrogen sensing, the Pt/TiOx film may revert to a more metallic state via the reduction of TiOx. The chemical interactions of these sensing films may be associated with large scale microstructural changes. The sensing activity and the structure of Ti, Pt/Ti, and Au/Ti films to gases such as hydrogen, and propylene has been studied. These chemical and morphological changes can be monitored by measuring the resistance of the thin sensing films. Pt/Ti films were found to have the best response to hydrogen (200 ppm) and propylene (150 ppm), while Au/Ti films showed weaker response to propylene (500 ppm) and CO (1000 ppm). The Pt/Ti films also showed shifts in temperature with changing propylene concentration possibly due to catalytic combustion.