A new intumescent flame-retardant (IFR) system consisting of expandable graphite (EG) and ammonium polyphosphate (APP) was applied in acrylonitrilebutadienestyrene (ABS) resin. A synergistic effect between EG and APP on the flame retardancy of ABS was observed. Fixing the total loading of flame retardant at 15 wt %, the limited oxygen index (LOI) could reach 31 vol % at a weight ratio of 3 : 1 for EG and APP. While LOI values of EG- and APP-filled ABS were only 26.0 and 21.5 vol % at the same loading, respectively. The UL-94 vertical burning test suggested that samples with different ratios of EG and APP could all pass V-0 rating while the samples containing EG and APP alone only passed V-1 rating. Thermogravimetric analysis indicated that the addition of EG and APP (3 : 1 by weight) to ABS led to an increase in the amount of high-temperature residue by 11.8 wt %, and a decrease of mass loss rate by 0.7%/degrees C compared with pure ABS. Scanning electronic microscopy revealed a homogeneous compact intumescent char layer of ABS/EG/APP samples. Based on our experiment and combined with others' previous studies, the synergistic mechanism is inferred. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012