A series of La-promoted Ru/zeolite X catalysts (similar to2 wt% Ru) was examined in the ammonia synthesis reaction at 20.7 atm total pressure. Results were compared with those reported previously for analogous Ba-promoted Ru/zeolites. Exchange of the zeolite cations with La-3+ formed an Ru catalyst (Ru/LaX) that was superior to one exchanged with Ba-2+ (Ru/BaX). Incorporation of La-3+ and Ba-2+ beyond ion-exchange capacity improved the turnover frequency on Ru/LaX and Ru/BaX, respectively. However, an La-promoted sample proved to be the most active. Interestingly, all of the La-containing catalysts exhibited a weak dependence of the rate on H-2 whereas the Ba-loaded samples were strongly inhibited by the reactant. These results suggest that an appropriate choice of promoter might overcome the well-known inhibition on Ru catalysts by H-2 without sacrificing high reactivity.