The Bureau of Mines has investigated the so2 leaching of mgo from high-mgo-bearing Florida phosphate concentrates. The so2 was effective in leaching some of the mgo from three phosphate concentrates; flow rate of the so2 and leach time were major factors in determing the amount of mgo leached. Calcining the samples, then grinding to minus 200 mesh, and elevating the leach temperature gave only small increases in the amount of mgo leached. The research also showed that some of the mgo was so intimately associated with the phosphate mineral that it could not be removed by so2 leaching. This amounted to about 0.6, 0.7, and 1.0 pct mgo in three leached residues. The calculated heads for the three concentrate samples averaged 1.4, 1.1, and 1.5 pct mgo, respectively, before leaching. This report is based upon work done under a memorandum of agreement between the Bureau of Mines, U.S. Department of the Interior, and the University of Alabama.