Novel heterogeneous catalysts from calcium oxide (CaO)/calcined calcium carbonate (CaCO3) loaded onto different palm oil mill boiler ashes were synthesised and used in the transesterification of crude palm oil (CPO) with methanol to yield biodiesel. Catalyst preparation parameters including the type of ash support, the weight percentage of CaO and calcined CaCO3 loadings, as well as the calcination temperature of CaCO3 were optimised. The catalyst prepared by loading of 15 wt% calcined CaCO3 at a fixed temperature of 800 degrees C on fly ash exhibited a maximum oil conversion of 94.48%. Thermogravimetric analysis (TGA) revealed that the CaCO3 was transformed into CaO at 770 degrees C and interacted well with the ash support, whereas rich CaO, Al2O3 and SiO2 were identified in the composition using X-ray diffraction (XRD). The fine morphology size (<5 mu m) and high surface area (1.719 m(2)/g) of the fly ash-based catalyst rendered it the highest catalytic activity. (C) 2012 Elsevier Ltd. All rights reserved.