Abstract: This report points out that chloride ion is a poison for iron catalyst of ammonia synthesis promoted by alkali metal. Results of the experiment on adding KCl to the system Fe-AC (active carbon)-Kshowed a very marked decrease in catalytic activity (Fig. la), while the addition of KCl to the system, Ru-AC-K, activity decreases much less (Fig. lb), indicating that chloride ion is a poison for iron catalyst but has little effect on ruthenum catalyst for ammonia synthesis.Gas chromotographic study on Fe₂(C₂O₄) S-AC and Fe₂(C₂O₄)₃-KCl-AC systems showed that in hydrogen atmosphere the decomposition of the iron salt took place at different temperatures. In the system that contained KCl, the decomposition products COand CO₃ appeared at a higher temperature (Fig. 2a, 2b). In hydrogen atmosphere, the reduction of FeOfrom pure Fe₂(C₃O₄)₃ crystals was almost completed at 300-420℃, but FeOof the Fe₂(C₃O₄)₃-AC system was reduced only slightly even at 500℃. Under the same condition, FeOfrom the Fe₂(C₃O₄)₃-AC-KCl system was reduced much more as indicated by the amount of water evolved (Fig. 2c). It was therfore postulated that the presence of KCl lowered the degree of dispersion of FeO and made it more easily reduced to form large particles of a-iron of low or no catalytic activity.X-ray diffraction study of the catalysts by the powder method revealed that the product of the decomposition of Fe₂(C₃O₄)₃-KCl-ACgave the characteristic spectrum of a-iron(Fig. 3) while that of Fe₂(C₃O₄)₃-ACgave no spectrum. It was also found, by comparing the Mossbauer spectra of the two systems, that in the catalyst from Fe₂(C₂O₄)₃-KCl-ACsystem the spectral lines of the fine particulate a-iron in paramagnetic region which corresponded to high catalytic activity disappeared.In conclusion, it is considered probable that the poisonous effect of chloride ion is due to the agglomeration of the superfine particles of a-iron which are the active centers of the catalyst for ammonia synthesis and ruthenum catalyst is much less sensitive to this agglomeration effect.