Using acid-catalyzed esterification, a continuous reactor, containing four separate continuous stirred tank reactors (CSTR's), was designed and used to reduce the free fatty acid (FFA) content of mixed crude palm oil (MCPO). A six-blade disk turbine and four baffles were installed in each of the four reactors to enhance mixing. The complete reactor was tested using response surface methodology (RSM). A 5-level, 4-factor, central composite design (CCD) was employed to optimize the four important reaction variables (methanol/oil ratio; sulfuric acid/oil ratio: speed of the stirrer; and residence time) to reduce the FFA content of the MCPO to less than 1 wt.% of oil. multiple regression analysis was used to derive a polynomial equation to predict the FFA content of the product. This was then used to indicate optimal conditions for reducing the FFA in mixed crude palm oil to less than 1 wt.%. Gravity separation of the contaminated water produced in the reaction was found to be sufficient to produce a feedstock for transesterification into ester. (C) 2009 Elsevier B.V. All rights reserved.