To evaluate the effect NH3 and NOx have on the performance of molten carbonate fuel cells (MCFCs), bench-scale cell tests and half-cell experiments have been performed with fuel gas containing NH3, or with oxidant gases containing NOx. Most of the added NH3 is discharged from the anode, and does not affect the cell voltage. The NO, does harm to the cell voltage during earlier operating stages, but the harm tends to decrease with increasing operating time. The main cause for the cell voltage drop is the increase of internal resistance. As a result of the analyses regarding the electrolyte composition in the operated cells, the gas composition and the cyclic voltammograms, the behavior of NOx in the cell is found to be as follows: NOx reacts with the carbonate and dissolves in the electrolyte to make NO2- and NO3-. These ions react with the hydrogen in the fuel gas and lead to the production of N-2 and a small amount of NH3. Consequently, NO2 and NO3- are not accumulated in the electrolyte, and the effect of NOx on the cell life expectancy is slight.