Mullite ceramics with controlled microstructure in terms of grain size/shape, pore and glassy phase content were produced from sol-derived pastes using extrusion. Particular attention has been given to the development of a continuous process which is suitable for the preparation of high-solids-loading mullite pastes from two different starting mullite precursors, namely, diphasic and molecular mixed mullite sols. A combined processing technique comprising vacuum filtering and pressure filtration was introduced in order to obtain extrudable mullite pastes from low solids-loading colloidal sols. It is shown that glassy phase free stoichiometric 3:2 mullite (3Al(2)O(3).2SiO(2)) with fine (0.94 mum) equiaxed grain microstructure is achievable from monophasic precursors after pressureless sintering at 1400degreesC for 3 h using the developed technique which can control both the sol-derived paste microstructure and process parameters. It is also found that the room and high temperature (1300degreesC) flexural strength and toughness of extruded mullites are mainly controlled by the grain size, the presence and location of glassy phase, nano-inclusions and pores at the grain boundaries. Pressureless sintered mullite derived from the monophasic sol-derived pastes provides flexural strength values of 345 and 277 M Pa for room temperature and 1300degreesC, respectively. (C) 2003 Kluwer Academic Publishers.