In this study, the fire performance and toxicity of intumescent flame retardant (IFR) polystyrene composites were investigated experimentally. Ammonium polyphosphate, pentaerythritol, and melamine were selected as IFR. The flammability of the polystyrene (PS) composites was evaluated by microscale combustion calorimetry and cone calorimetry and in the ISO Room. The results suggested that the thermal stability and the peak heat release rate of PS composites decreased with the increasing content of IFR. In the cone calorimeter and ISO 9705 testing, the carbon monoxide yield of PS composites also decreased markedly with the addition of IFR. Scanning electron microscope images show that the char from cone calorimetry testing was more compact and smoother than that from the ISO 9705 testing. The comparison between bench- and full-scale tests demonstrated that the flammability and the toxicity of PS composites are decreased markedly due to the incorporation of the flame retardant, but considerable differences exist. Copyright © 2014 John Wiley & Sons, Ltd.