Preparation of TiB2-Al2O3 and NbB2-Al2O3 in situ composites with a broad range of phase composition was conducted by self-propagating high-temperature synthesis (SHS) involving thermite reactions of different types. Thermite mixtures of Al-TiO2 and Al-TiO2-B2O3 were incorporated with the Ti-B combustion system to produce the composites of TiB2-Al2O3, within which the increase of the thermite mixture for a higher content of Al2O3 decreased the reaction temperature and combustion wave velocity. This implies that the thermite reaction of Al with TiO2 reduces the exothermicity of the overall SHS process. In the synthesis of NbB2-Al2O3 composite. two thermite mixtures of Al-Nb2O5 and Al-Nb2O5-B2O3 were added to the Nb-B combustion system and both of which were found to increase the combustion temperature and propagation rate of the flame front. This is due to the highly exothermic nature of the thermite reaction between Al and Nb2O5. For both kinds of composites, it was found that adoption of B2O3 as one of the thermite reagents improved the product formation effectively. The XRD analysis shows that the final products composed of no more than TiB2 and Al2O3 are obtained from the powder compacts containing the thermite mixture of Al-TiO2-B2O3. On formation of the NbB2-Al2O3 composite, NbB2 is identified as the major boride phase in the products involving the thermite reactions of Al-Nb2O5-B2O3, while Nb3B4 dominates in the case of using Al and Nb2O5 as the thermite reagents. (C) 2009 Elsevier B.V. All rights reserved.