Polymerization of a di-isocyanate with the amine-modified surface of a sol-gel derived mesoporous silica network crosslinks the nanoparticles of the silica skeleton, and reinforces the otherwise fragile framework. Systematically adjusting the processing variables affecting density produce aerogels whose macroscopic properties could be controlled, and are attributed to changing nanoscale morphology. Aerogels crosslinked using the smallest amount of silica studied exhibit as much as a 40-fold increase in strength over the corresponding non-crosslinked framework, and are flexible. (c) 2006 Elsevier Ltd. All rights reserved.