Data on durability, petrography and microstructure are presented for 27 specimens of Waikato Coal Measures mudrocks: fine-grained, carbonaceous rocks, which are dominated by kaolinite clays, and have low durabilities. The poor durability is not caused by swelling clays. Rather, slaking is believed to result from the release of residual stresses within the rock following weakening by water adsorption on to clay surfaces and a consequent loss in cohesiveness. This mechanism favours fragmentation into small, water-stable fragments, rather than dispersion of clays into the water. The amount of day-sized material in the rocks provides the most direct control on the durability, but mierostructural features are also recognized as important: specimens with a discontinuous matrix have a slightly lower durability than those with a continuous matrix.