The kinetics of nitriding of iron and iron-phosphorus alloys in ammonia-hydrogen gas mixtures have been investigated at 580°C. The rate of nitriding of pure iron was found to be consistent with that predicted assuming nitrogen diffusion in the solid to be the rate limiting step and this condition was also observed over a range of oxygen potentials under the same nitriding conditions. The presence of phosphorus in the iron was found to reduce nitriding rates. The rate limiting reaction in the nitriding of Fe-P alloys in ammonia-hydrogen gas mixtures at one atmosphere total pressure is determined to be NH2(adsorbed) → NH(adsorbed) + H(adsorbed). A first order reaction rate equation is developed which predicts the nitriding rate to be inversely proportional to the bulk phosphorus content of the alloy at a given hydrogen partial pressure.