The flame retardancy mechanisms of a novel polyhedral oligomeric silsesquioxane containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-POSS) in polycarbonate (PC) composites are discussed. A nice flame retardancy performance of 6 wt % DOPO-POSS/PC is detected from cone calorimeter testing. The peak heat release rate and total heat released of 6 wt % DOPO-POSS/PC decrease obviously compared with that of PC. The major pyrolysis products detected from the decomposition process of PC and 6 wt % DOPO-POSS/PC are investigated by TGA coupled with Fourier transform infrared spectrometry and mass spectrum (TGA-FTIR and TGA-MS), respectively. It is confirmed that DOPO-POSS catalyzes thermal decomposition of PC, however, reduces the release of flammable volatiles during the decomposition of PC. The condensed phase products after decomposition of PC and 6 wt % DOPO-POSS/PC are investigated by FTIR, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy characterizations. They indicated that DOPO-POSS can accelerate the formation of char during the decomposition of PC composites. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012