Laboratory tests were conducted to study removal of magnesium impurities from western phosphate rock concentrates by selective leaching. The systems evaluated included leaching with dilute sulfuric acid, leachng with ammonium sulfate, and leaching in an aqueous medium with sulfur dioxide gas. Set standards for the magnesium impurity level of phosphate rock concentrates are not available, but industrial practice has indicated that magnesium in excess of 0.3 percent mgo causes problems in phosphoric acid manufacture. Removal of magnesium was accomplished with all three systems. With dilute h2so4 the magnesium level was reduced to less than 0.1 percent mgo with an accompanyng p2o5 loss of 6 percent. Magnesium oxide levels from 0.3 to 0.2 percent were obtainable with ammonium sulfate. However, the system requires concentrate roasting at temperatures of 900 deg. C or higher, heated pulps (100 deg. C), and large quantities of ammonium sulfate. Selective leaching was obtained using so2 gas in an aqueous concentrate slurry. Magnesium oxide content was reduced to less than 0.1 percent without loss of phosphate.