Formation of open-cage oligomeric azidophenylsilsesquioxane was surprisingly discovered in the successive reduction and azidation reactions starting from octa(nitrophenyl)octasilsesquioxane. The mixture of oligomeric products was characterized after end-capping with trimethylsilyl groups. The IR spectra confirmed the introduction of azide function to aromatic moiety, and trimethylsilyl group on silsesquioxane framework. Together with the GPC result, NMR analysis revealed the introduction of different number of trimethylsilyl groups in the structure. Some of them maintained the cage-like silsesquioxane structure characterized by XRD analysis. Their TGA profiles gave a unique pattern with clear two-step mass loss with the first mass loss of about 8-10 wt% from 180 to 225 degrees C corresponding to a thermal decomposition of azide groups. (C) 2010 Elsevier B.V. All rights reserved.