Abstract A new type of optical sensor for the detection of gases is proposed. The working principle is as follows: the gas to be measured is absorbed by a sensitive film, which changes its refractive index with gas absorption. The sensitive film is deposited onto an integrated optical interferometer. As the intensity distribution of waveguide modes is not totally confined to the waveguide, the interferometer phase changes with the refractive index of the film. Tests have been made with interferometers of Fabry-Pérot and Mach-Zehnder type. Organically modified silicates are used as sensitive films. Films sensitive towards, e.g., CO 2 and SO 2, have been tested under different gas atmospheres. The temperature dependence of the sensor signal and cross sensitivities towards other gases and humidity have been investigated. A sensor element has been fabricated that will be suitable for the detection of the threshold limit value of CO 2.