The response of the kinetics of surface-catalyzed reactions to different initial conditions in a gas nitriding furnace has been studied. The predictions are made under the conditions with both high and low concentrations of ammonia and a process temperature of around 520 A degrees C. Results are presented in temporal variations of gas temperature and concentrations and coverages of surface species. It has been shown that in the composition where NH3 content is richer, there is a strong possibility of developing substantially higher N-ad coverage and so diffusion into the material. Reducing the ammonia content, which means an increase in the proportion of N-2 in the initial mixture, leads to a quicker saturation of the furnace atmosphere to the desorption of the reaction product, N-2, which eventually suppresses the advance of reaction in the forward direction. Low coverage values of H-ad have proved that the adsorbed hydrogen is the most unstable species on the surface.