This paper reports the results of studying ion-exchange synthesis of chlorine-free potassium sulfate from KCl and Na2SO4 on strong acid cation-exchange and strong base anion-exchange resins using an ion-exchange isothermal supersaturation (MISS) effect. This effect is observed for a number of ion-exchange systems, where the ion-exchange process is accompanied by the formation of a stable supersaturated solution of slightly or sparingly soluble substances in the interstitial space of ion-exchange columns. The formation of stable K2SO4 solutions in the columns has been observed by the displacement of either K+ or SO42- ions from a cation or anion exchanger with concentrated Na2SO4 or KCl solutions, respectively. After leaving the column, a supersaturated solution crystallizes spontaneously, which allows for the design of a practically ideal ion-exchange process where a crystalline chlorine-free potassium sulfate is obtained right after the ion-exchange conversion cycle. The process is practically wasteless and ecologically clean; hence, it can be considered as a competitive alternative to existing K2SO4 production technologies based on, e.g., the Mannheim process. The flow sheet of a proposed process is presented and discussed.