A polystyrene/bis (2, 4-dicumylphenyl) pentaerythritol diphosphate (DPP) composite (PS-DPP) with a DPP loading of 10% (w/w) was prepared by the melt-compounding method. Results indicated that DPP had a thermal destabilization effect at temperatures below 380 degrees C. A comparative degradation kinetic analysis was performed employing the Kissinger method and the isoconversional methods of Friedman, Starink and Advanced Isoconversional Method (AICM). Variation of activation energy with extent of conversion (alpha) results from all the isoconversional methods showed that activation energy did not vary significantly with a for both pure PS and PS-DPP. Activation energies obtained from the Starink method above 380 degrees C were very close to those obtained from AICM consistent with activation energy values not varying significantly with a. The y(alpha) master plots and the Friedman methods where used to identify the reaction model and calculate the frequency factor respectively. The Sestak-Berggren model was identified as the most appropriate model to describe the thermal degradation of both PS and PS-DPP. The overall results indicates that activation energies of the composite were not significantly higher than those of pure PS at low conversion (alpha < 0.2) where DPP had a destabilizing effect but became higher when DPP had a stabilization effect.