Porous ceramic filters have applications in many industrial areas. The major requirements for filter materials are high porosity, high mechanical strength and uniform pore size. However, conventional fabrication methods cannot produce materials with both high porosity and high strength, since an increase in porosity usually results in a decrease in strength. A method has been reported to obtain porous materials of high strength and high porosity by hot isostatic pressing (HIP), because the high pressurised gas prevents shrinkage of open pores and enhances bridging of particles. In the present study several commercial Al2O3 starting powders were uniaxially and isostatically pressed and the green compacts were directly treated under different HIP conditions. The microstructure (grain size and shape, density and pore size distribution) of the HIP products was evaluated and then compared to that of the traditionally sintered ones in order to investigate the influence of starting powder characteristics, forming process and thermal treatment parameters on the characteristics of the final materials.