A laser gas monitoring system has been developed to measure in-situ concentrations of gaseous pollutints (in our case ammonia) at various temperatures. The heart of the system is a tunable 13CO2-waveguide laser. By varying the resonator length the laser is frequency modulated between two neighboring emission lines. One line is absorbed by the species investigated, while the other is used as a reference. By this referencing the system is autocalibrated continously. The high spectral resolution of the laser method supresses interference by other species. Fast evaluation of the digitalized signal shortens the measurement time to less than one minute. The lasers high output power is used in a multiple path arrangement, which allows the detection of ammonia concentrations in the one vppm range. First results of measurements in a power plant are reported.