Shock-recovery and shock-spallation experiments were performed on two compositions of aluminium-infiltrated B4C cermets as a function of shock pressure. Sixty-five per cent volume B4C-Al cermets were recovered largely intact after shock loading up to pressures of ca. 12 GPa which permitted a critical study of the microstructural changes produced by the shock. Significantly, shock loading to between 12 and 13 GPa produced a combination of dislocation debris, stacking faults and deformation twins in a small fraction of the B4C grains. Fragmentation of shock-loaded 80% B4C-Al samples prevented meaningful microstructural investigation. Spall-strength testing also provided indirect evidence for the Hugoniot elastic limits (HEL) of these composites. Spall-strength calculations based on an elastic equation of state for 65% B4C-Al indicated that the elastic regime extended up to shock pressures of ca. 10 GPa, or approximately 65% of the HEL of polycrystalline B4C. A complete loss of spall strength was then observed at the transition to a plastic equation of state at a pressure of 12 GPa which coincided with observations of plasticity within the B4C-substructure. This study demonstrated that composites containing a highly ductile phase combined with a high compressive strength ceramic phase could support high dynamic tensile stresses by resisting the propagation of catastrophic cracks through the brittle ceramic substructure.