This paper is focussed on the reinforcing phases developing in a Si3N4-based composite microstructure when sintering a powder mixture of 74 wt.% Si3N4, 20 wt.% TiO2, and 6 wt.% Y2O3. Micron- and submicron-sized as well as Al2O3 and Al2O3 + SiO2-coated submicron-sized TiO2 powders were used to prepare these in situ composites consisting Of Si2N2O, alpha-Si3N4, beta-Si3N4, and TiN. The impulse excitation technique was used to evaluate the intergranular glass phase, which affects crack growth and high-temperature deformation. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to investigate the reinforcing phases. The influence of the TiO2-additive powder characteristics on the microstructure and properties of the ceramics is reported. (C) 2003 Elsevier Science B.V. All rights reserved.