Clay-based mullite was prepared for insulating substrate applications. The clay and reactive alumina are the starting materials for the stoichiometric mullite prepared by reaction sintering at 1600 °C/3 h. MgO and boehmite were added as sintering aid to improve the density and decrease the sintering temperature. The properties of the sintered bodies, such as thermal, electrical and mechanical properties of the sintered samples were studied. The boehmite (5.0 wt.%) substituted for reactive alumina mixture showed higher density (96% theoretical density). Clay and reactive alumina completely reacted at 1600 °C and formed mullite phase. Samples with 3.0 wt.% MgO addition showed the presence of secondary phases such as α-alumina and spinel. The mullite crystals formed are needle-shaped with rectangular faces. The MgO (3.0 wt.%) addition slightly decreased the thermal expansion. Dielectric constant values are almost same for the samples with boehmite or MgO addition. © 2003 Elsevier B.V. All rights reserved.