Much interest has been taken in finding a solid base catalyst for a reaction to produce biodiesel. Calcium oxide has the great advantage of the enhanced catalytic activity, but the soluble substance is leached away from the solid base catalyst during the reaction. In this paper, the leaching of solid base catalyst was investigated on the basis of data from the heterogeneous catalytic transesterification of soybean oil at reflux of methanol. When calcium oxide was employed for the reaction, the calcium contents of the produced oil and glycerol were 139 and 4602 ppm, respectively. This data indicated that the amount of the soluble substance corresponded to 10.5 wt% of the employed catalyst. Since calcium oxide was transformed into calcium diglyceroxide at the beginning of the reaction, many of the soluble substances derived from calcium diglyceroxide. Also, the soluble substances were rather active in the soybean oil transesterification. On the other hand, calcium diglyceroxide was employed for the reaction, with the result that the amount of the soluble substance was only 4.0 wt%. In this case, the soluble substance did not catalyze the conversion of soybean oil into its methyl esters. Based on the experimental results, the heterogeneous catalysis of calcium oxide was discussed. Additionally, removal of the soluble substance by cation-exchange resin was tested in order to purify the produced oil. (C) 2008 Elsevier B.V. All rights reserved.