Abstract: A steam and gas turbine combined cycle plant STAG* employs a catalyst in a heat recovery steam generator to react injected ammonia with NO.sub.x from the combustor of the gas turbine to reduce atmospheric emission of NO.sub.x from the system. Rapid control of ammonia injection is achieved using a prediction of the NO.sub.x being generated in dependence upon the operating conditions of the combustor. A trimming signal from a measurement of NO.sub.x being emitted from the heat recovery steam generator downstream of the catalyst is employed to complete the NO.sub.x control loop. Compensation is provided to relate the measured NO.sub.x downstream of the catalyst with the NO.sub.x in the output of the gas turbine taking into account the catalyst efficiency.