Fermentation kinetics of butyrate production from glucose and xylose by Clostridium tyrobutyricum ATCC 25755 immobilized in a fibrous-bed bioreactor at pH 6.0 and 37 degreesC were studied. Fermentation with glucose as the main carbon source gave higher cell biomass production and reactor productivity compared with xylose fermentation, probably due to the extra energetic requirement for cell growth on xylose. Acetate formation was also reduced when xylose was the substrate. However, comparable butyrate concentration of similar to 40 g/l and butyrate yield of 0.43 g/g were obtained from both glucose and xylose fermentations. In the fermentation of glucose/xylose mixture (1:1), the acid production pattern from xylose did not change in the presence of glucose and no inhibition of xylose uptake was observed. Corn fibre hydrolysis with dilute HCl or H2SO4 at 121 degreesC produced similar to 62% (w/w) fermentable sugars, mainly xylose, glucose, and arabinose. The hydrolysis was not significantly affected by the acids used, their concentrations between 0.1 and 0.5 M, or the treatment time between 15 and 60 min. Butyrate fermentation of corn fibre hydrolysate supplemented with corn steep liquor gave a high butyrate yield of 0.47 g/g carbon source consumed and a reactor productivity of 2.91 g/l h This study demonstrates that the low-value corn refining byproducts can be efficiently used for butyrate production. (C) 2002 Elsevier Science Ltd. All rights reserved.