The emission of nitrogen oxides from combustion can be limited by selective catalytic reduction (SCR) and selective non-catalytic reduction (SNR) by ammonia. In order to control the ammonia slip in these processes it is desirable to measure the ammonia outlet concentration continuously. The problems in doing this are discussed, among which are homogeneous and heterogeneous reactions of ammonia with nitric oxide in the sampling system. The design and test of a diluting flue gas probe is presented, where ammonia is converted to and detected as nitric oxide. The efficiency of various converter designs for the probe was studied in the laboratory.