Abstract: An oxygen sensor is based on a thin film of a compound oxide supported on a substrate such as quartz. The compound oxide has a general formula ABO.sub.2.5+x, where A is an element of the lanthanum family, alkaline earth metal or their mixture, and B is a transition metal or a mixture of transition metals. X is a variable number, changing from about 0 to about 0.5 between the reversible oxygen-deplete and oxygen-rich forms of the oxide. In an elevated temperature and oxygen-containing atmosphere, the thin film can undergo reversible bulk oxidation resulting in a detectable change of its physical properties such as mass, optical transmissivity and electrical resistance.