A previous paper presented the results of measuring the ammonia (NH3) concentration in the combustion chamber of a circulating fluidised-bed (CFB) boiler. Two sampling techniques were used: a gas-quenching (GQ) probe where the sample is quenched directly in the probe tip by a circulating trapper solution, and a gas-sampling probe connected to a Fourier-transform infra-red (FTIR) instrument. In this paper, an attempt is made to estimate the influence of potential secondary reactions with NH3 in the GQ probe tip by means of a plug-flow model. The calculations showed that heterogeneous and homogeneous reactions could be of importance to the accuracy of the measurement. This contradicts the almost identical results relative to the parallel FTIR measurement, and the conversion of NH2 is probably overestimated by the model. In addition, transients obtained during the FTIR measurement were used to study the accuracy of the gas-sampling probe.