A novel intumescent flame retardant (IFR), containing ammonium polyphosphate (APP) and poly(tetramethylene terephthalamide) (PA4T), was prepared to flame-retard acrylonitrile-butadiene-styrene (ABS). The flame retardation of the IFR/ABS composite was characterized by limiting oxygen index (LOI) and UL-94 test. Thermogravimetric analysis (TGA) and TGA coupled with Fourier transform infrared spectroscopy (TG-FTIR) were carried out to study the thermal degradation behavior of the composite and look for the mechanism of the flame-retarded action. The morphology of the char obtained after combustion of the composite was studied by scanning electron microscopy (SEM). It has been found the intumescent flame retardant showed good flame retardancy, with the LOI value of the PA4T/APP/ABS (7.5/22.5/70) system increasing from 18.5 to 30% and passing UL-94 V-1 rating. Meanwhile, the TGA and TG-FTIR work indicated that PA4T could be effective as a carbonization agent and there was some reaction between PA4T and APP, leading to some crosslinked and high temperature stable material formed, which probably effectively promoted the flame retardancy of ABS. Moreover, it was revealed that uniform and compact intumescent char layer was formed after combustion of the intumescent flame-retarded ABS composite. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012