The synergistic effects of phosphorus-silicon in a caged bicyclic phosphate quadridentate silicon complex (CPQS) on the flame retardancy of epoxy resin (EP) were studied with the limiting oxygen index, the UL-94 test, thermogravimetric analysis, real time Fourier transform infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy. The limiting oxygen index data, UL-94 test, and thermogravimetric analysis results showed that the use of phosphorus and silicon together had a synergistic effect on the flame retardancy of EP. The Fourier transform infrared results and X-ray powder diffraction measurements provided evidence that CPQS could promote the formation of char with silicophosphate and phosphocarbonaceous structures. The morphological structures observed by scanning electron microscopy demonstrated that more compact charred layers were formed in samples with CPQS than in those with 1-oxo-1-phospha-2,6,7-trioxabicyclo[2.2.2]-4-hydroxymethyl octane and tetraethoxysilane. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 168-174, 2009