Modern adsorption study facilities as well as scanning electron microscopy, X-ray diffraction, Mossbauer spectroscopy and X-ray fluorescence spectroscopy were used to investigate the effect of cobalt on the adsorption of nitrogen, hydrogen and ammonia on the surface of cobalt modified iron fused catalysts. Adsorption studies were carried out at the temperature range specific for ammonia synthesis (385 degrees C). Activity tests were carried out under 10 MPa in the 350-450 degrees C temperature range. Investigations were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Introduction of cobalt changed considerably the sample behaviour during activation and ammonia synthesis. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approximately 5.5 wt% Co. Cobalt changed the adsorption behaviour of the catalyst. Chemisorption of nitrogen is much higher for "cobalt" catalysts. Growth of nitrogen chemisorption and decrease of ammonia adsorption resulted in the growth of catalytic activity of "cobalt" catalysts in ammonia synthesis.