Corn. nanocryst. Fe catalyst for NH(3) synthesis was reduced with H(2) at 500 degrees C and then oxidized with H(2)O at 350-500 degrees C (partial pressure 0.0087-0.0123 bar) to passivy the catalyst surface. Three catalysts with grain sizes 0.75-1.0, 1.0-1.2 and 1.5-2.5 mm were studied sep. by thermogravimetry. At low conversion degree (below 0.2), the oxidn. proceeded in the kinetic range (activation energy 22 kJ/mol) independently on the catalyst grain size. At higher conversion degrees (above 0.5), the oxidn. proceeded in the diffusion range. The reaction mechanism was proposed.