Samples of representative Illinois limestones and dolomites were ground in distilled water and the amount of water-soluble salts in the resulting leaches was determined by chemical analysis and by weighing the leach solids resulting from evaporating the leaches. The dolomite leaches contained more soluble salts on an average than the limestone leaches and commonly contained the more Mg (super ++) , HCO 3 (super -) , and Cl (super -) .The limestone leaches generally exceeded the dolomites in Ca (super ++) and SO 4 sup (super --) . X-ray diffraction data on the leach solids from the limestones showed that they are generally characterized by the presence of calcium sulfate, calcium carbonate and sodium chloride. Distinctive compounds of dolomite leach solids are magnesium chloride and basic magnesium carbonate; magnesium sulfate and potassium chloride are found more commonly than in limestone leach solids.The leaches from dolomites were generally slightly more basic than those from limestones. The calcium sulfate in the leaches may come largely from the solution of inter-crystal deposits of this compound, but the other salts probably come mostly or entirely from intra-crystal fluid inclusions. These inclusions are smaller but more numerous in the dolomites than in the limestones.Conclusions regarding the occurrence of the water-soluble salts are based on a combination of evidence from thin sections, decrepitation tests, fetid limestone, and efflorescence data.Water soluble salts have potential economic significance in connection with lime-burning and the physical properties of lime, the uses of lime-stone and dolomite powders, and the durability of building stone.