The internal structure of oolitic limestones of varying durability from quarries in England and the United States has been examined by mercury porosimetry, optical and electron microscopy and crystallization experiments with sodium sulfate. Porosimetry measurements exhibit hysteresis which have been related to durability. Microscopic examination reveals three classes of void structure: 1) large voids between randomly packed, spherical oolites; 2) voids and cleavage cracks in the polycrystalline structure of calcium carbonate that, in some stones, fills the void between the oolites; 3) submicroscopic voids in the structure of the oolites. These microstructures are related to durability. See also AATA 19-1198 and 21-914.