A gelcasting process has been successfully employed to fabricate porous mullite-corundum ceramic composites from natural clay and corundum powders. The specimens based on the mullite composition are found with an open porosity of 45-47.9%, mean pore size of 1.28-2.55 mum, and nitrogen permeability of 965-5038 m(3) . m(-2) . bar(-1) . hr(-1) by reactively sintering the gelled mixture of kaolinite and alpha -alumina at 1100-1500 degreesC. The open porosity (rho (o)), mean pore size (d(p)), pore size distribution and gas permeability can be controlled by adjusting raw material ratios and sintering temperatures. The gas permeability of the specimens is found to be more dependent on the pore size distribution as well as d(p) than on rho (o). In addition, the gas transportation mechanism in porous mullite-corundum ceramic composites is dominated by viscous flow.