This paper presents a modelling study of gaseous pollutants (CO, NO, N2O, SO2, HCl) formation and destruction during the incineration of municipal solid waste in a circulating fluidized bed combustor. To describe axial voidage profiles, a model which calculates the bed density as an exponential function of the bed height is used. The bed is divided into a dense region at the bottom where gas and solids are perfectly mixed and a dilute zone above considered as a plug flow. The model includes devolatilization of waste particles, combustion of char and volatile matters, acid gas recapturing by calcium, NO and N2O formation and reduction by homogeneous and heterogeneous reactions. The impact of different operating parameters (temperature, excess air, calcium addition, waste moisture and air stagement) was studied and the results of calculations were compared with the emissions measured on a CFB pilot plant of 10 cm i and 4 m height. The predictions are in good agreement with experimental data.