Using expandable graphite (EG) and ammonium polyphosphate (APP) as flame retardants, we prepared two series of polyisocyanuratepolyurethane (PIRPUR) foams (i.e., EG foams filled with different amounts of EG alone and APEG foams containing different amounts of expanded EG and APP) and evaluated the effect of the additives on the physicalmechanical property, fire behavior and thermal stability of the foams based on compressive strength test, limiting oxygen index (LOI), cone calorimeter test, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The addition of EG alone or both EG and APP into the foam greatly influences the physicalmechanical property. The compressive strength of APEG foams is closely related to the apparent density. The LOI value showed good improvement in both EG and APEG foams. The addition of APP in APEG foams gave better fire behavior than the EG foams with an obvious decrease in PHRR and increase in residue. In addition, the TGA curves illustrated that APP might be an effective charring agent to promote char formation. The SEM results showed that the incorporation of APP and EG allowed the formation of a cohesive and dense char layer, which inhibited the transfer of heat and combustible gas and increased the thermal stability of PIR-PUR. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013