In the first part of the paper we present the experimental validation of models describing the NOx removal from flue gases through the SNCR process. The Selective Non-Catalytic NOx Reduction process was realized in a high temperature flow reactor. A lead salt diode laser system combined with second derivative spectroscopy monitored the concentration of NO, NH3, N2O, NO2, HCN behind the reactor. After the injection of ethane at concentrations of up to 1000 ppm the shift (up to 220 K) of the temperature window of the SNCR process was measured.In the second part a Laser In-Situ Ammonia-monitor (LISA) based on a 13CO2-wave guide laser is used to establish an on-line control of the SNCR in an industrial environment by measuring in-situ the excess ammonia behind the reduction zone. With differential absorption spectroscopy and a special referencing scheme sensitivities in the order of several ppm·m of NH3 at atmospheric pressure have been achieved even in heavily dust laden flue gases. With a multi-channel LISA system the 2-dimensional NH3 distribution in the flue gas channel of an experimental coal burner system could be measured and the persistence of spatial inhomogeneities, caused by incomplete mixing of NH3 and the flue gas could be shown.In the third part we present first results aimed at the measurement of the local O2-concentration in combustion systems. With a thermally stabilized 5 mW NIR diode laser at 760 nm we detected oxygen at atmospheric pressure with a sensitivity of less than 500 ppm·m using two beam absorption spectroscopy.