Abstract: The main aim of this work was to investigate the synergistic effect of expandable graphite (EG) and aluminum hypophosphite (AHP) on the flame retardancy of rigid polyurethane foams (RPUFs). A series of flame retardant RPUF containing EG and AHP were prepared by one-shot and free-rise method. The flame retardant, thermal degradation, and combustion properties of RPUF hybrids were characterized through limiting oxygen index (LOI) test, vertical burning (UL-94) test, thermogravimetric analysis and microscale combustion calorimeter. The LOI and UL-94 results showed that the RPUF sample with 10wt% EG and 5 wt% AHP passed UL-94V-0 rating and reached a relatively high LOI value of 28.5%, which is superior over other EG/AHP ratios in RPUF at the equivalent filler loading. Microscale combustion calorimeter results revealed that the incorporation of EG and AHP into RPUF reduced the peak heat release rate and total heat release, thus decrease the fire risk of RPUF significantly. Incorporation of EG and AHP improved the thermal stability of RPUF as observed from the thermogravimetric analysis results and also enhanced the thermal resistance of char layer at high temperature from scanning electron microscopy and Raman spectroscopy. Moreover, it could be seen from thermogravimetric analysis/infrared spectrometry spectra that the addition of EG and AHP significantly decreased the combustible gaseous products such as hydrocarbons and ethers. Finally, the synergistic mechanismin flame retardancy was discussed and speculated. Copyright (C) 2014 John Wiley & Sons, Ltd.