Abstract: In this study, determination of material balance for reciprocal salt pair systems is considered and special case of KNO3 production is solved. A scheme is presented for calculation of material balance which is based on the conservation of ions and water. Material balance requires equilibrium composition and phase diagram of solid-liquid equilibria that is calculated by extended UNIQUAC thermodynamic model. Hence, composition of streams in process was calculated. Based on ion conservation in crystallizers, a simple and handy method was presented to calculate flow rate of all salts and water.