A laboratory scale model reactor was used to investigate volatile fatty acid (VFA) formation during anaerobic degradation in an anaerobic hybrid reactor. The purpose of the study was to determine the role of upflow anaerobic sludge blanket (UASB) and anaerobic filter (AF) regions of the hybrid reactor on bio-degradation of organic compounds using VFA as an operational parameter. Acetic, propionic, butyric, and valeric acid concentrations of samples taken from the reactor effluent, fixed bed region and the upper part of the sludge bed region were analysed. VFA formation was monitored at two phases. In phase-I, 10kg COD m(-3) per day organic loading rate (OLR) (C-o = 10,000 mg COD l(-1) and theta(H) = 1 day) and in phase-II 7.6kg COD m(-3) per day OLR (C-o = 15,000 mg COD l(-1) and theta(H) = 2 days) was applied. The average acetic acid concentrations of the upper end of the reactor and sludge bed region were 150 and 354.9 mg l(-1), respectively. These results showed that VIA concentrations increase from the top to the bottom of the reactor, indicating higher methanogenic activity in the upper part and higher acetogenic activity in the bottom of the reactor. pH measurements confirmed this result. (C) 2003 Elsevier Ltd. All rights reserved.