Process control measures are of critical importance which guard against process instability of anaerobic digestion. By introducing perturbations in an up-flow anaerobic filter reactor fed with olive mill wastewaters (OMWs), reliable parameters that could suggest process imbalance were determined. The reactor was operated for a period of 10 days at 37degreesC and at an HRT of 23 days corresponding to a loading rate of 0.75 g COD l(-1) per day. Changing the organic loading rate (OLR), hydraulic retention time (HRT) and temperature lead to reactor performance failure. A decrease in biogas production and methane yield was observed together with an accumulation of VFAs such as acetate, propionate, butyrate, isobutyrate, and valerate. Significant concentrations of butyrate, isobutyrate, and valerate accumulated when acetate and propionate concentrations were very high. The pH at the bottom of the reactor, where substrates were introduced, showed a low pH value (acid), whereas no such change was observed at the top of the reactor. Valerate accumulation was shown to be the most toxic of all VFAs but an accumulation of propionate, butyrate and acetate also inhibited the functioning of the reactor. (C) 2004 Elsevier Ltd. All rights reserved.