The need of more efficient solid catalysts for the heterogeneous slurry Fenton process led many investigators to research new compounds activities. Ceria based iron catalysts have proved their good performance enhancing the removal of organic compounds, reducing toxicity and improving biodegradability in the deputation of phenolic wastewaters. This work evaluates the calcination temperature (300 degrees C, 600 degrees C and 1000 degrees C) during the preparation of the catalysts co-precipitation of the precursors salts, while the same previously optimized conditions were adopted (pH 3.0,1.0 g L-1 of Fe-Ce-O 70/30 as the catalyst, [H2O2] = 244 mM and 120 min of room temperature reaction) to treat a simulated wastewater comprising 0.1 g L-1 of each of the six common phenolic acids found in Olive Mills wastewaters. The three obtained solids were characterized regarding superficial area, average pore diameter, FT-IR and XRD. Catalysts calcinated at 300 degrees C, 600 degrees C and 1000 degrees C presented superficial areas of 188,86 and 2 m(2)/g, respectively, and their average pore diameter are 66.87 and 151 angstrom, correspondingly. As showed in the XRD, the increase of the calcination temperature promotes the crystallinity of the obtained solid higher amount of prominent peaks, meaning that the catalysts have different states of valence for iron (Fe2+. or Fe3+) and ceria (Ce2+ to Ce4+), what would explain their singular behaviours during the reaction. As expected, the solids with higher superficial areas had better performances in every aspect: more COD, TOC and phenolic acids removal, pointing the lowest calcination temperature as leading to a more efficient solid to enhance hydrogen peroxidation, involving, however, more metal leaching. The higher the calcination temperature is, the more oxidized the solid will become because the calcination occurs without atmospheric control and so the oxygen contained in the air will interfere on the valence of iron ions at the solid's surface. This means that a solid calcinated at a higher temperature will have increased Fe3+ content and, as one can find in the literature. Fe2+ is more effective than Fe3+ at hydrogen peroxidation, explaining the better efficiency of the catalyst calcinated at the lowest temperature. Toxicological and biodegradability studies were still performed and showed enhancement in all cases. (C) 2011 Elsevier B.V. All rights reserved.