The chemical form of the potassium promoter on an iron-alumina catalyst during ammonia synthesis has been studied by two methods, viz, (i) the measurement of the equilibrium constant of the process KNH, + He = KH + NH,, and (ii) chemical analysis of the used catalyst. The equilibrium constant measurements gave KTza = (12.9 k 0.5) X 10m3, AHl,,(KNH,) =-I 19 * 3 kJ mol-' and S,O,,(KNH,) = 109 ? 4 J molV' K-'. The chemical analysis showed that no KNHZ is present on the catalyst during synthesis. From these results and with the aid of thermodynamic considerations it is concluded that KNH,, K and K,O are not stable compounds under conditions of ammonia synthesis. X-Ray diffraction showed that part of the potassium reacts with A&O,, probably leaving part of the potassium in the form of KOH which is quite stable under ammonia synthesis conditions.