The performance and the characteristics of a laboratory scale anaerobic baffled reactor (ABR) were investigated using synthetic wastewater. The experimental results showed that among different volatile fatty acids (VFAs), acetate was the main intermediate of acidogenic degradation of glucose: The VFA concentration decreased longitudinally down the reactor. The analysis of the biogas composition revealed that methane concentration increased steadily from compartment I to 5, while hydrogen content decreased in the first compartments. There was no detectable hydrogen in the last two compartments. The methane-producing activity of anaerobic sludge in different compartments depended on the substrate, which suggests that the proper anaerobic consortium in each separate compartment was developed according to the substrate(s) availability and the specific environmental conditions. The ABR has the potential to provide a higher efficiency at higher loading rates and be applicable for extreme environmental conditions and inhibitory compounds. (C) 2003 Elsevier Ltd. All rights reserved.