This study was conducted to determine thermal decomposition of gypsum (CaSO4 . 2H(2)O) and celestite (SrSO4), to examine the effect of heating on grindability and the possibility of celestite-gypsum separation by differential grinding after heating. The results indicated that no significant decomposition reaction was observed in celestite up to 900 degreesC whereas the decomposition of gypsum occurred within 100-450 degreesC. Work indices (W-i) of celestite and gypsum were found as 6.76 and 5.18 kWh/shton, respectively. W-i of celestite did not change significantly after heating at 200 degreesC where that of gypsum decreased to 4.00 kWh/shton. The difference in grindabilities makes it possible to separate celestite from gypsum by differential grinding, This method has the advantage that celestite-gypsum beneficiation and production of plaster of Paris are achieved in a single stage. (C) 2003 Elsevier Ltd. All rights reserved.