The plate impact loading response of three aluminas with varying glass content (purity) and porosity has been investigated using the 50 mm gas-gun facility at the University of Cambridge. The elastic precursor amplitude has been shown to be sensitive to both shock propagation distance and impact stress. The precursor decay effect is greatest in the low purity aluminas. The effect of the material microstructure on the compressive strength at high rates of strain has been studied. Porosity was shown to play the dominant role although some grain-size effects were also noted.