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The influence of the mineralogical composition and phase distribution on crack initiation and propagation in cordierite-mullite refractory materials used in fast firing of porcelain whiteware is investigated. Two different refractory compositions, characterised by different silica to alumina ratios, were studied. Propagation of cracks introduced by Vickers’ indentations was observed by scanning electron microscopy. Chemical analysis by EDS was used for phase identification together with X-ray diffraction analysis. Thermal and mechanical properties were determined by means of standard laboratory techniques. The fracture toughness of selected samples was measured by the chevron notched specimen technique. Preliminary assessment of thermal shock resistance was obtained by water-quenching tests from 1250ºC. Microstructural features and crack propagation behaviour were correlated and used to draw conclusions on the behaviour of the two different refractory compositions under thermal shock.