A set of clay shales associated with coal measure strata has been examined to determine the mechanisms that lead to slaking on immersion in water. The two main mechanisms for the slaking of small unconfined samples are (a) compression of entrapped air and (b) osmotic swelling of expansive clay minerals if the dominant exchangeable cation is sodium. In the absence of osmotic swelling, pore-air compression can induce slaking in clay shales of bulk densities less than 2.27 Mg/m3. Osmotic swelling of clay shales occurs in samples with bulk densities less than about 2.45 Mg/m3, but slaking by this mechanism occurs only for samples with densities less than 2.34 Mg/m3. Dispersion of clay from clay shales develops only after slaking commences, and the extent of dispersion depends on the nature of the interparticle bonds.Key words: clay shales, clays, coal measure rocks, slaking, dispersion, porosity, density.