A new experimental method has been proposed enabling the determination of the effect of reduction degree (R) and concentration of water vapour (CH2O) on the activation rate (r=dR/dt) of the iron catalyst for ammonia synthesis. A wetless mixture of H-2 and N-2 (3:1) was introduced into the reaction chamber of an automated flow thermobalance. The applied reactor can be considered as a reactor with ideal mixing. A monolayer of catalyst grains was reduced in wet atmosphere produced by reduction itself. The reduction degree was calculated from the loss of mass recorded during the reduction. The measured reduction rate and a known fixed value of the flow rate of the gas mixture enabled us to calculate the concentration of water vapour. The experiments differ from each other by the initial mass of the catalyst and by the flow rate. The variability of these factors resulted in the variation of CH2O within the industrially important range of 1000-10 000 ppm. The final reduction degree varied from 78% to 95%. The results of the series of experiments performed at ca. 550 degrees C were presented in the form of a 3-D diagram : r=r(R, CH2O) The diagram cut by planes at cH(2)O=const yields curves r=r(R). These curves were described by a Gauss-like function. The diagram cut by plane R=10% yields the initial rate curve r=r(cH(2)O). The curve was described by an exponential function. The model function was proposed in order to describe the 3-D diagram, too. The acquisition of data by the proposed method is relatively easy, especially because there is no need, similarly as in the industrial installations, to saturate gases with water vapour.