The performance of the several different modified mordenite zeolites in the degradation of expandable polystyrene waste (EPSW) was investigated in this study. The composition of the degradation products, especially the liquid fraction was compared to understand the role of H-mordenite and the effect of its modification through framework dealumination and metal impregnation. The degradation was carried out in a batch reactor at 360 and 400degreesC. The dealuminated mordenite with SiO2/Al2O3 ratio of 86 showed enhanced styrene/styrene dimer and styrene/ethylbenzene molar ratios at lower (360degreesC) and higher (400degreesC) temperature, respectively. This was attributed to the intermediate acid strength distribution generated on removal of framework aluminum by acid leaching. Modification of H-mordenite by metal impregnation substantially changed the acidic and catalytic properties in polystyrene degradation. The 0.64 wt.% phosphorous loading on H-mordenite showed improved EPSW conversion, styrene yield and SM/EB molar ratio at 400degreesC. The 0.32 wt.% phosphorous loading facilitated the improvement in SM/EB molar ratio with higher styrene selectivity than thermal degradation at 360degreesC. (C) 2004 Published by Elsevier B.V.