Novel halogen-free compounds [9,10-dihydro-9-oxa-10-phosphaphanthrene-10-oxide/vinyl methyl dimethoxysilane/N-beta-(aminoethyl)-gamma-aminopropyl methyl dimethoxysilane (DOPO-VMDMS-NMDMS)] that simultaneously contain phosphorus, nitrogen, and silicon have been synthesized through the reaction of 9,10-dihydro-9-oxa-10-phosphaphanthrene-10-oxide (DOPO), vinyl methyl dimethoxysilane (VMDMS), and N-beta-(aminoethyl)-gamma-aminopropyl methyl dimethoxysilane (NMDMS). The chemical structure and properties of DOPO-VMDMS-NMDMS have been investigated with Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, phosphorous nuclear magnetic resonance, and thermogravimetric analysis (TGA). These synthesized flame retardants have been blended with a poly[2,2-propane-(bisphenol) carbonate]/acrylonitrile butadiene styrene (PC/ABS) alloy. The flame-retardant properties of these mixture samples have been estimated with the limiting oxygen index (1,01), and the thermal stability has been characterized with TGA. The LOI value of PC/ABS/DOPO-VMDMS-NMDMS is enhanced up to 27.2 vol % from 21.2 vol %, and the char yield is also improved slightly (from 12 to 17%) with 2.8 wt % phosphorus, 3.0 wt % silicon, and 0.5 wt % nitrogen (at a 30 wt % loading of DOPO-VMDMS-NMDMS). The results show that there is a synergistic effect of the elements phosphorus, silicon, and nitrogen on the flame retardance. (c) 2007 Wiley Periodicals, Inc.