The economic feasibilities of four continuous processes to produce biodiesel, including both alkali- and acid-catalyzed processes, using waste cooking oil and the 'standard' process using virgin vegetable. oil as the raw material, were assessed. Although the alkali-catalyzed process using virgin vegetable oil had the lowest fixed capital cost, the acid-catalyzed process using waste cooking oil was more economically feasible overall, providing a lower total manufacturing cost, a more attractive after-tax rate of return and a lower biodiesel break-even price. On the basis of these economic calculations, sensitivity analyses for these processes were carried out. Plant capacity and prices of feedstock oils and biodiesel were found to be the most significant factors affecting the economic viability of biodiesel manufacture. (C) 2003 Elsevier Ltd. All rights reserved.