The pore systems of 7 oolithic limestones of known durability have been examined by mercury porosimetry. The measurements have been extended beyond the first intrusion curve to include the volume of mercury retained in the stone as a function of increasing and decreasing pressure from 1 to 1000 atmospheres. This procedure generates a hysteresis loop, the area of which corresponds to the energy (P x V) required to fill and partially empty the pore system. This energy, which is independent of structural models of the pore system, ranges from 1000 to 9000 joules per gram for the 7 stones which represent the most durable Portland stone and the least durable Bath stone.