Catalytic activity of calcium oxide as a heterogeneous catalyst was investigated for transesterification of sunflower oil to methyl esters. The primary objective of this study was to investigate the catalyst activity (biodiesel yield) as a function of its active phase and textural properties imposed by different activation temperatures: 500, 700, 800, 900, 1000 and 1100 degrees C. Therefore the catalyst activity, basicity, textural and bulk properties were correlated. X-ray diffraction analysis (XRD) showed Ca(OH)(2) next to CaO phase in all samples activated up to 900 degrees C. Their fraction changed, however, with activation temperature increase, leading to pure CaO in samples activated above 900 degrees C. The same samples, however, performed with the lowest activity in the series, in which the catalyst sample calcined at 800 degrees C showed the maximal activity. Simultaneous thermal analysis (TG-DTA) indicated two-step dehydroxylation of CaO, suggesting different fractions but also positions of hydroxyl groups upon calcination at different temperatures. Both Fourier transformed infrared spectroscopy (FTIR) and temperature programmed desorption (TPD) of CO2 preadsorbed on CaO surface indicated the sample activated at 700 degrees C with the highest basicity. It is unique in the series containing basic sites of both strong and medium strength. High basicity of that sample, however, happened not to be enough for the highest activity, possibly due to poor textural properties, as revealed by results of low temperature nitrogen adsorption (LTNA). Maximal specific surface area, total pore volume and dominant fraction of a high-diameter mesopores were found in the case of the sample activated at 800 degrees C. Thus, a complex function of active sites of medium strength, possibly induced by a specific hydroxyl groups coverage, as well as mesoporosity directing an unrestricted mass transfer, resulted in the maximal activity of the CaO sample activated at 800 degrees C. (C) 2015 Elsevier Ltd. All rights reserved.