The activities of single step sol-gel made calcium oxide on alumina catalysts were studied as a function of CaO loading, methanol/oil molar ratio and the amount of the catalyst in the transesterification of sunflower oil at 50 degrees C. Also, the turnover frequency (TOF) of the catalysts was calculated to better understand the relationship between the basicity/basic strength and the catalytic activity. From volcano curve (TOF vs. basic strength), it was found that under 50 degrees C and methanol/oil molar ratio of 9, 60% CaO/Al2O3 had the highest turnover frequency, 0.028 s(-1), whereas 85% CaO/Al2O3 showed the highest biodiesel yield, similar to 96.6%, but TOF obtained on it was 0.012 s(-1). It seemed that 60% CaO catalyst had the proper basic strength to obtain the highest TOF. (C) 2011 Elsevier Ltd. All rights reserved.