BET, scanning electron microscopy, X-ray diffraction, Mossbauer spectroscopy, X-ray fluorescence spectroscopy and McBain thermobalance were used to investigate the effect of cobalt on the reduction behaviour and activity of fused iron catalyst for ammonia synthesis. Activity tests were carried out under 10 MPa in the 350-450 degrees C temperature range. Studies were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approx. 5.5% wt Co. Cobalt changed the reduction behaviour of the catalyst. The rate of the surface change during reduction was higher for the case of the ’cobalt catalyst’; however the rate of mass change was higher for a typical iron catalyst. The process of reduction was probably followed by the formation of an Fe3Co compound and by the surface faceting, with the exposure of an Fe(111) plane.