A kinetic description is given of the high-pressure ammonia synthesis reaction over a Fe based catalyst. The description is based on a statistical mechanical treatment of the competitive adsorption of the reactants, intermediates, and product on identical surface sites. The kinetic expression with parameters taken from ultrahigh vacuum for the reactants on single-crystal surfaces is able to describe the observed ammonia kinetics for an industrial catalyst over a very broad range of conditions (1–300 atm, 375–500 °C). This provides a direct verification that the ‘‘pressure gap’’ between surface science and heterogeneous catalysis can be bridged. Further, the present description gives a detailed picture of the working catalyst. Variations in surface coverages down through the reactor bed and the origin of the activation enthalpy of the overall reaction are discussed. We also show that the microscopic picture of poisoning and promotion whereby the effect of surface impurities is to stabilize or destabilize the adsorbing molecules can account quantitatively for the observed promotion and poisoning of the high-pressure ammonia synthesis by potassium and water, respectively.