A novel, hydroxy-functional, organophosphorus flame retardant (FR), 2,2-dihydroxymethylpropane-1,3-diolylbis(hydrogen phenylphosphonate) (DHDBP), was synthesized and characterized by Fourier transform infrared (FTIR) spectroscopy, H-1-NMR, C-13-NMR, P-31-NMR, and elemental analysis. Subsequently, poly(ethylene terephthalate) (PET)/cotton (T/C; 70/30) blends were treated via pad-dry-thermosol finishing with DHDBP, citric acid, and a catalyst. Its flame retardancy, durability effect, and thermal decomposition behaviors were investigated by limited oxygen index, vertical burning test, thermogravimetric analysis, FTIR spectroscopy, and scanning electron microscopy. The results show that DHDBP was not only a reactive FR with a high efficiency but also a good char-forming agent for the T/C blends. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3066-3074, 2010