Nanoparticles of tungsten trioxide (WO3) were synthesized using sol-gel chemistry and combined with nanoscale aluminum (Al) particles to form a thermite. The as-made sol-gel-derived WO3 aerogel contains impurities such as hydroxyl groups that are inherent to the synthesis process. These impurities can be removed via heat-treating the powder, and both as-made and heat-treated mixtures were examined for ignition and flame propagation as a function of the mixture bulk density. Results showed that the hydroxyl impurity impedes flame propagation in high density (compressed pellets) by acting as a heat sink and absorbing energy during flame propagation. In loose powders (low density mixtures), convection plays a more dominant role in flame propagation, such that the reduced particle size of the as-made WO3 produces higher flame propagation speeds than the enhanced thermal transport properties associated with the heat-treated WO3.