Powder mixtures of silicon nitride (Si3N4) With 6 wt%. Y2O3 and containing up to 63 wt% TiO2 were hot-pressed. The obtained ceramics are investigated using high temperature X-ray diffraction, the impulse excitation technique, and scanning electron microscopy. It was found that during hot pressing, the TiO2 additives were converted to TiN, and silicon oxynitride (Si2N2O) was formed. The addition of TiO2 was found to enhance the alpha to beta-Si3N4 conversion during sintering. A decreased thermal expansion of the crystal structure of the beta-Si3N4 phase is observed with increasing TiO2 amount. The influence of the TiO2 content of the powder mixture on the microstructure and mechanical properties of the densified ceramics was elucidated and the observed reaction mechanisms are thermodynamically and kinetically analyzed. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.