Supercritical carbon dioxide (SC-CO2) as a medium for esterification of free fatty acids (FFA) of hydrolyzed soy deodorizer distillate (DOD) with butanol, has been studied using Mucor miehei lipase as a biocatalyst. Process conditions (pressure, temperature, butanol and enzyme concentration and incubation time) were optimized by conducting experiments at five different levels using a central composite rotatable design (CCRD) method. A second order polynomial response surface equation was developed, indicating the effect of variables on ester yield. Contour maps generated using the response surface equation, showed that all the experimental variables significantly affected the yield of fatty acid butyl esters (FABE). The reaction variables influenced the yield of esters both independently and in combination. A pressure of 122 bar, temperature 36degreesC, butanol concentration 1.2 M, enzyme concentration 15% (w/w) and incubation time of 3 h were predicted to be the optimal for the maximum yield of 100% FABE, which was validated by an experiment to yield 95% of FABE. SC-CO2 is a potential medium for the biomodification of soy DOD by esterification of free fatty acids with butanol. (C) 2004 Elsevier B.V. All rights reserved.