Barium-promoted Ru/HTAC (hydrogen-treated active carbon) is reported to be a promising ammonia catalyst. However, deactivation and regeneration have been observed under some conditions. In this study, the activity change of this catalyst was studied in detail. The effects of the reaction temperature, Ba(NO3)(2) decomposition temperature, and the water vapor pressure on the ammonia-synthesis activity were investigated to elucidate the cause. It was found that the promoter components under reaction conditions were Ba an Ba(OH)(2), of which the molar ratio varied with the temperature and water vapor pressure, obeying the reversible reaction BaO + H2O = Ba(OH)(2). It was concluded that the activity was changed reversibly depending on the chemical form, BaO or Ba(OH)(2). The stronger promoting effect of BaO vs. Ba(OH)(2) was attributed to its stronger electron donation to Ru. The activity drop at 588 K of the sample activated at 823 K was concluded to be due to the decrease of the BaO portion resulting from the contained water vapor (10 Pa is the threshold at 648 K). At high temperature, deactivation can not occur because of the thermodynamic equilibrium of BaO/Ba(OH)(2). For alkali metals, their hydroxides (CsOH or K) are not considered to turn to the active-oxide phase under the usual ammonia-synthesis conditions.