Waste cooking oil (WCO) was purified by supercritical carbon dioxide (scCO(2)) extraction and catalytic hydrodeoxygenation of fresh soybean oil, WCO and scCO(2)-purified oil were studied for the synthesis of diesel-like hydrocarbons. The chemical compositions of the scCO(2)-purified oils were very similar to those of fresh oil. Hydrotreating of WCO over the Pd and Ni catalysts showed lower conversions when compared to the NiMo and CoMo catalysts. The hydrotreating of scCO(2)-purified oil using a Pd/Al2O3 catalyst resulted in 85.7% conversion, which is comparable to the 91.0% conversion from fresh oil whereas only 41.8% of WCO was converted under identical conditions. Hydrotreating of the scCO(2)-purified oil with other catalysts including Ni/SiO2-Al2O3, CoMo/Al2O3 and NiMo/Al2O3 resulted in similar conversions in the range 84.4-92.9% compared to that from fresh oil. The scCO(2)-purification was effective for removing the various impurities in WCO, that were responsible for the detrimental effects on catalytic activity especially for the Pd and Ni catalysts. The origin of catalyst deactivation caused by the impurities in WCO is discussed. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.