The IPI-POSS-modified epoxy resin (IPEP) was prepared from isocyanato-propyldinethylsilyl-isobutyl-POSS (IPI-POSS) and diglycidyl ether of bisphenol A epoxy resin. The steric hindrance of the IPEP bulky POSS side chain improved the curing activation energies. The POSS particles sizes were about 2-3 nm and dispersed uniformly. At lower IPEP concentration (POSS < 12 wt %), the glass transition temperatures (T(g)s) of the IPEP nanocomposites increased from 118 to 170 degrees C. The char yield increased from 15 to 20 wt %, and the LOI values increased from 22 to 28. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 643-652, 2010