The activation methods to improve the activity, basicity and stability of CaO as the catalyst for the transesterification of soybean oil have been investigated. It was determined that calcium glyceroxide complex is formed during the transesterification and acts as the most active phase. Moreover, it was observed that the catalytic activity of spent calcium oxide after transesterification was significantly increased comparing to the fresh catalyst. Thus, activation with glycerol is more efficient technique for the increasing of the CaO transesterification activity compare to the traditional techniques such as thermal treatment and activation with methanol. The synthesized calcium glyceroxide catalyst showed the highest catalytic activity. Under the study conditions, with a methanol to oil molar ratio of 9:1 and a reaction temperature of 60 degrees C 82.6% FAMEs yield was obtained within 120 min with only 1.3 wt.% (respect to the oil mass) of calcium glyceroxide. At same time, the FAMEs yield was only about 76.9% when CaO activated with methanol was used. (C) 2014 Elsevier B.V. All rights reserved.