Many researchers focused their attentions on calcium oxide (CaO) as a solid base catalyst for transesterification of vegetable oil with methanol, but, a small amount of soluble substance is leached away from CaO catalyst under the reacting condition. In the present research work, purification of biodiesel contaminated with the leached CaO catalyst was studied. Biodiesel was prepared by the CaO-catalyzed transesterification of waste cooking oil, and calcium content of the prepared biodiesel was 200 ppm. The purifying test to remove the leached CaO catalyst was carried out by passing the prepared biodiesel through a column packed with cation-exchange resin. Among the employed four resins differing in the polymer matrix, the acidic functional group and the counter-cation, the protonated resins consisting of macro-reticular polymer matrix resulted in the high removal efficiency. Then, an influence of the internal mass transfer on the removal efficiency was investigated on the basis of data collected by purifying the biodiesel-methanol mixture. The addition of methanol promoted internal mass transfer, so that the 100% removal of the leached CaO catalyst was achieved successively. Furthermore, reduction of the free glycerol was examined for the resin-purification. (C) 2012 Elsevier Ltd. All rights reserved.