Ammonia synthesis and decomposition have been performed over a doubly promoted (Al2O3, CaO) and a triply promoted (Al2O3, CaO, K2O) fused iron catalyst at atmospheric pressure, Additionally, the temperature-programmed surface reaction (TPSR) of N-ads with H-2 has been studied over both catalysts. No effect of potassium on the turnover frequency of NH3 synthesis was found, On the contrary, a significant fivefold increase in the rate of NH3 decomposition in the presence of potassium was observed. The effect was found to be strongly dependent on the concentration of NH3 in the gas phase, The promoting mechanism of potassium is discussed in relation to the surface structure of the catalyst under varying operating conditions.