The effect of rigid inclusions on the densification during isothermal sintering of glass matrix composites was investigated. Mixtures containing borosilicate glass powder and 0, 5, 10 and 25 vol. % alumina (Al(2)O(3)) particles were prepared and powder compacts isostatically pressed at 200 MPa have been used. The sintering behaviour of the samples heated at 800'C during various times was investigated through density measurement, axial and radial shrinkage measurements. The microstructure was analysed by SEM and the crystalline phases present in the sintered composites were identified by XRD. The relative density of the isothermally treated borosilicate glass-Al(2)O(3) composites decreased significantly with the increase in Al(2)O(3) content because the presence of rigid inclusions retarded the densification of the compacts. The borosilicate glass exhibited anisotropic shrinkage behaviour, showing a radial shrinkage higher than the axial shrinkage and isotropic shrinkage was favoured by Al(2)O(3) additions. Sintered glass showed a dense microstructure with some spherical closed pores. The microstructure of composites with 5 vol. % Al(2)O(3) revealed that most of the pores were filled by capillary flow of the glass. The microstructure of composites with higher Al(2)O(3) additions showed dense areas together with interconnected pores, which appeared at the sites of large glass particles in the green compacts.