The oxidized form of a model, fused iron catalyst for ammonia synthesis, containing large amounts of wustite (ca. 40 wt.-%), was activated by reduction. The reduction of catalyst samples with different thermal history and different grain size was continuously monitored by thermogravimetry as well as periodically by Mossbauer spectrometry. Prior to the reduction, the samples were annealed in nitrogen atmosphere, The longer the time of primary annealing was, the faster was the reduction process. Some conclusions could be drawn about the nature of the acceleration - the induction period seemed to be shorter, whereas the reduction mechanism seemed to remain unchanged, The wustite phase disproportionates during the initial annealing, forming metallic iron and magnetite as the final products. Magnetite originating from the decomposition of wustite showed a faster reduction rate than the primary magnetite.