The effect of Hot Isostatic Pressing (HIPping) treatments on porosity in the aluminium casting alloy A357 and stir-cast A357/15 vol% SiC particulate Metal Matrix Composite has been investigated. Densitometry and image analysis have been used to determine the apparent percentage porosity. Four HIPping temperature profiles have been investigated (103 MPa pressure) and mechanical testing has been carried out by single notch four point bending. The bend strength was increased after HIPping relative to as-received. The optimum HIPping cycle involved HIPping for a short time in the semi-solid region followed by a sustain below the solidus.