The crystallization of K2SO4 in the system K+, Mg2+/Cl-, SO42/H2O involving the reaction 2KCl + MgSO4 reversible arrow K2SO4 + 2KCl was studied. Independently of the experimental procedure applied, it was confirmed that schoenite (K2SO4.MgSO4.6H(2)O) is an inevitable intermediate product of the considered transformation. The double decomposition reaction kinetics depend on formation and dissolution rates of intermediate salt and potassium sulphate, respectively. The formation of schoenite outside of its crystallization field might indicate a metastable space extension of this domain to cover the K2SO4 one. The formation kinetics rate of MgSO4.K2SO4.6H(2)O is quicker than the K2SO4 one.