Five ammonia synthesis catalysts, mainly differing in potassium content, were prepared from a commercial doubly promoted iron catalyst. The activities of these catalysts were measured at 350–450 °C and 5–200 atm. The experimental reaction rates were fitted to the modified Temkin rate equation. Increasing the potassium content from 0.1 to 3.8 wt% results in increasing the order in H2 from 0.7 to 1.5. The change from singly to doubly promoted behavior is gradual. The nature of catalysts with a relatively high K-content is changed, preventing the formation of NH (or NH2) groups so that N-atoms become the main species on the surface, which explains the higher order in H2.