Ruthenium supported on gamma-alumina modified with KOH was tested as catalyst for ammonia synthesis under atmospheric pressure. Ru-3(CO)(12) and RuCl3 were compared as ruthenium precursors. The catalysts derived from Ru-3(CO)(12) exhibited an up to five times higher productivity than the corresponding ones derived from RuCl3. Ruthenium metal dispersion, which probably influences the catalytic activity, was measured both by carbon monoxide and hydrogen chemisorption tests, and observed by high resolution transmission electron microscopy (HRTEM), and was clearly higher in the case of the ruthenium cluster. Different ruthenium catalysts, with 2, 5, 8 and 10% metal and 30% potassium (added as KOH) on alumina, were prepared from Ru-3(CO)(12). The highest hourly productivity of ammonia, both referred to unit mass of the catalyst and to the ruthenium atom content, was obtained with 8% Ru/Al2O3-KOH at 623 K and atmospheric pressure.