Aim of this work is to investigate how SO2 and NOx can affect MCFC performance. Experimental tests will be performed at the Fuel Cell Center laboratories of KIST and ENEA thanks to 10 cmχ10 cm single cell and 3 cm2 button cell facilities and using a gas composition which simulate conditions of MCFCs when running in a combined cycle power plant. In the case of the button cell a different composition was used in order to avoid a fast degradation of the cell related to corrosion of current collector and fluctuation of EIS spectra. Polarisation curves, endurance tests, impedance measurements and gas analyses will be carried out to support the investigation. Theoretical analysis will be based also on a modeling experience which allows the use of detailed codes validated on MCFC standard operation without contaminants and which are now being to be extended to operation in presence of contaminants on the basis of the experimental data. Both experimental and theoretical results will be presented and discussed in detail.