Optical diagrams of waveguide sensors for measuring concentrations of components in gas mixtures and liquids are considered. The characteristics of the absorption cells of several waveguide systems are calculated as functions of parameters of optical waveguide. An ammonia waveguide sensor based on a Ta2O5 film deposited on a glass substrate is considered. The sensitivity of the sensor is 6 × 10-6. Methods for increasing the sensitivity of the sensor are suggested.