The effects of porosity on cracking during thermal shock have been studied by directly observing the cracks that formed after quenching heated porous alumina bars into water. The porosity was introduced by adding different volume fractions of fugitive inclusions and the behaviour compared with that obtained by partial sintering of a powder compact. Where fugitive inclusions had been used, there was little effect of either pore size or pore volume fraction over the ranges studied. The extent of cracking was always slightly less than that of a monolithic, dense alumina and gave reasonable agreement with predictions using experimentally measured data. However, cracks grew much further in the partially sintered material. This discrepancy became greater as the temperature change increased, inconsistent with existing analyses. It is suggested that this difference in behaviour arises predominantly because of the greater measured fracture energy of the alumina made using fugitive inclusions compared with that made by partial sintering.