The paper summarizes our results published for many years and also adds new information. Kinetic data concerning the reduction of the oxidized forms of model as well as industrial catalysts used in ammonia synthesis were obtained in dry and in wet atmosphere containing 1% water vapour. The data for the industrial catalyst have been reassessed using three kinetic models. Modifications applied to the classical Seth-Ross model of the shrinking-core type resulted in the best fitting of this equation to the experimental data. The failure of the crackling core model to describe the kinetic data in a quantitative way is tentatively explained. The reduction of model catalysts containing enhanced amounts of wustite and/or potassium proceeds initially linearly with time. The effect of promoters, and especially of potassium, is discussed in more detail. The magnetite-alumina subsystem is responsible for the retardation effect of the water vapour on the reduction rate. A hypothesis is formulated that - in the presence of wustite or potassium - the inhibitive, Al-rich, hydrated surface layer is not effective in hindering the progress of the reduction process.