In order to obtain high ethanol yield and fermentation rate, response surface methodology (RSM) was employed to study the effect of culture medium on the ethanol productivity from stalk juice of sweet sorghum by immobilized yeast. A 2(3) central composite design (CCD) was chosen to explain the combined effects of the medium constituents, viz. nitrogen (adjusted by adding (NH4)(2)SO4), phosphorus (adjusted by adding KH2PO4), and pH. A mathematical correlation about the influence of the nitrogen, phosphorus, and pH on the ethanol productivity was established. It predicted that the maximum ethanol production rate (119.12 g/l h) was observed for a medium consisting of 0.77 g/l phosphorus, 2.15 g/l nitrogen, and pH = 6.39. Under this condition, the ethanol fermentation rate was 122.85 g/l h. (C) 2008 Elsevier Ltd. All rights reserved.