A jet of clean potassium is continuously injected into hydrogen. A miniature electromagnetic pump is used to recirculate the molten metal. Rates of reaction of the gas with the metal surface over the pressure range 22·2–0·3 kN m–2 have been measured at temperatures between 210 and 333 °C. The reaction obeys first-order kinetics with an apparent activation energy of 66·5 kJ mol–1 which is slightly lower than that (72·4) for sodium. At the higher temperatures the process is complicated by the gradually increasing dissociation pressure of potassium hydride.