The solid state reaction between mica (phlogopite, muscovite) and calcium or magnesium sulphates or chlorides has been studied at elevated temperatures. The aim of the work was to obtain potassium in a soluble form by exchanging it by alkaline earth metal in mica. The addition of CaO, CaCO3 or MgCO3 promoted the recovery of potassium. When phlogopite, gypsum and calcium oxide were fired at 1100°C for 30 min, almost 90% of the potassium could be obtained in solution. The recoveries obtained with muscovite were significantly lower than those with phlogopite. The greatest amounts of soluble potassium were obtained by exchanging K+ ions with Ca2+ ions using CaCl2. In separate experiments with different muscovites and experimental conditions, 34–69% of the potassium content reacted to give a soluble form. The effect of the experimental conditions (reaction time, temperature, particle sizes, amount of accelerator) on the exchange reaction have been investigated.