Biodiesel is commonly produced by catalytic transesterification (reversible reaction) of oil/fat with excess alcohol. In the case of the basic catalyst, strong acid is usually used for catalyst neutralization; however acid converts soap to fatty acids (increases the acid number). In this paper, the utilization of carbon dioxide for catalyst neutralization is described. The advantage is that, the carbon dioxide does not transform soaps to fatty acids, but only neutralizes the catalyst (acid number is always less than 0.2 mg KOH/g). The neutralization takes place due to by an excess of CO2, which transforms the catalyst to hydrogencarbonate. However during the alcohol removal by distillation from the whole reaction mixture, hydrogencarbonate is transformed to carbonate (catalytic properties). But the alcohol removal is fast enough that the reverse reaction to glycerides does not occur. The catalyst neutralization by CO2 is environmentally friendly (consumed CO2) and it can be used for all types of raw materials, alkali hydroxides and alcohols. (C) 2012 Elsevier Ltd. All rights reserved.