Abstract One of the key points in the study of Ru/C catalysts for ammonia synthesis is the structure sensitivity of the reaction, which usually determines the proper metal loading, so governing the cost of the final catalyst. In the present work, the effect of Ru loading on catalytic activity and thermal stability (i.e. catalyst resistance against support methanation and metal sintering) was investigated. The optimal Ru loading was found to be around 3.5 wt.%, a value considerably lower with respect to that commonly found in literature. Furthermore, top activity was obtained with 10–15% Ru dispersion, higher dispersion values leading to less performing catalysts. Finally, the effect of some different water-soluble Ru precursors on catalyst behaviour was also investigated.