The influence of ammonium polyphosphate (APP) with and without synergistic agents - aluminum hypophosphite (Al(H2PO2)(3)), epoxy resin (E-44), and Zeolite 4A - in acrylonitrile-butadiene-styrene/poly(ethylene terephthalate) (ABS/PET) blends was investigated. The limiting oxygen index (LOT) and UL94 tests were used to investigate flame properties. The results showed that APP with and without synergistic agents all improved the flame retardancy of ABS/PET systems. The ABS/PET/APP + 2 wt Al(H2PO2)(3), ABS/PET/APP + 2 wt % E-44, and ABS/PET/APP + 3 wt % Zeolite 4A systems achieved the best flame retardancy rating of VU and LOT values were 32, 34, and 30.5, respectively. The thermal degradation behaviors analyzed by thermogravimetric analysis (TGA) indicated APP mainly accelerated the degradation of PET and led to the increase of charring residues. Al(H2PO2)(3) and E-44 resulted in synergistic effects on the charring process of ABS/PET/APP. SEM revealed that the intact, dense, and strong char layer formed on the surface of the ABS/PET/APP/ Al(H2PO2)(3) (or E-44) system, which was significantly different from the loose and porous char layer of ABS/PET and ABS/PET/APP systems. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1589 1597, 2010