The novel flame retardant tris(3,5-dimethyl-4-diphenyl phenoxy phosphate)isocyanurate (TDPI) was synthesized from the reaction of diphenyl chlorophosphate and tris(3,5-dimethyl-4-hydroxybenzyl)isocyanurate. The compound was characterized with Fourier transform infrared spectroscopy, elemental analysis, mass spectrometry, H-1-NMR, and P-31-NMR. Furthermore, blends of the flame retardant with engineering plastics in various weight ratios were made, and their thermal stability and flame retardancy were determined by thermogravimetric analysis (TGA), UL-94 vertical testing, and limiting oxygen index (LO1) measurements. The phosphorus-nitrogen synergistic effect on flame retardancy was compared with the effects of commonly used tetraphenyl resorcinol diphosphate and tetrabromobisphenol A. TGA indicated that the TDPI/engineering plastics systems were stable up to 397-503degreesC at a 5% weight loss in a nitrogen atmosphere with a relatively high char yield (21.0-42.1% at 700degreesC) and that they a achieved UL-94 V-0 rating (with a low flame-retardant content: P, 0.76 wt%, and N, 0.34 wt %). No fume and toxic gas emissions were observed during the burning test for this system. The LOI values were 31.6-42.2. (C) 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 470-417, 2004.