Abstract: A reactor for the catalytic synthesis of ammonia at high temperature and pressure from nitrogen and hydrogen wherein the intertubular spaces of the heat exchange zone are filled with metallic balls; the tubular spaces contain metal bars with square cross section; triple concentric tubes are present in the preheating and reaction zone to reach optimal temperature conditions; a demister is provided for the retention of oil at the entrance of the synthesis gas into the intertubular spaces of the heat-exchange zone; and a bag-shaped distribution chamber is provided between the reaction and preheating zone and the heat exchange zone.