K2O-promoted nanocryst. Fe (grain size 34 nm) was nitrided with NH3 in a differental reactor under atm. pressure at 400 degrees C. Thermogravimetric measurements and anal. of the gas phase compn. were carried out simultaneously. During the process, the nitriding potential changed until the stationary state was reached. The rate of catalytic NH3 decompn. was lower for the Fe sample with lower K2O content (0.1% by mass).