Short chain fatty acid fermentation was carried out using Lactobacillus buchneri ATCC 4005 under anoxic conditions at low pH values using xylose as substrate. Fermentation studies were conducted with free cells and with cells immobilized on alumina fibers in batch, fed-batch, and continuous reactors. The results indicate that xylose is readily utilized by L. buchneri as a carbon source with an estimated maximum specific growth rate (mu(max)) of 0.15 h(-1) for free cell and immobilized cell fermentations. Lactic and acetic acids were produced at high concentration levels of 67 g L-1 and 32 g L-1, respectively in fed-batch fermentation without any apparent cell inhibition. Volumetric productivities of 6.4 g L-1 h(-1) for lactic acid, and 4.3 g L-1 h(-1) for acetic acid were obtained at a dilution rate of 0.5 h(-1). The Luedeking-Piret model that relates cell growth to product formation is utilized to develop a mathematical model of the continuous fermentation system. The model predictions for volumetric acid productivities as a function of dilution rates are presented, and are shown to agree well with experimental data. This study is the first demonstration of a continuous fermentation process for the conversion of xylose to lactic and acetic acids using L. buchneri at a pH value of 4.0.