Abstract: Catalytic ammonia decomposition was studied  in order to obtain pure hydrogen for fuel cells, free of  COx that are being formed during reforming of hydrocarbons and alcohols. In comparison with hydrogen, ammonia is easy to store and transport, because it is liquid at the temperature of 25 °C under the pressure of 8 atm. In the presented studies a catalytic decomposition of ammonia over fused iron-cobalt catalysts was examined. Catalysts were prepared by alloying iron oxides with an addition of cobalt(II) oxide and promoters (Al₂O₃, CaO i K₂O). Catalysts with various contents of potassium oxide were tested. Chemical composition of the prepared catalysts were determined with an aid of XRF method, whereas phase compositions of the catalysts were established with XRD method. Also the catalysts were characterized by programmed temperature techniques: H₂-TPD, TPR. Measurements of the catalysts activity in the ammonia decomposition reaction were carried out in a differential reactor, which enabled to record mass changes and analyze the gas phase. Ammonia decomposition ran at an ambient pressure in the temperature range from 475 to 600^oC. In the inlet of the reactor ammonia concentration from 5 to 100 %vol. were tested. On the basis of the analysis of gas sampled from the space above the catalyst bed, nitriding potential P=p_NH3/p_H2^3/2 and an ammonia decomposition rate r[mol/g_cat ∙s] were calculated. An addition of potassium oxide into iron fused catalysts develops  their specific surface areas. An increase in potassium oxide content in the tested catalysts had a beneficial impact on their activity in the ammonia decomposition reaction.