The influence of the organic substrate composition in the feed of an innovative reactor, the periodic anaerobic baffled reactor (PABR) is examined. A laboratory-scale PABR fed on a synthetic medium composed of mixtures of glucose (a carbohydrate) and gelatin (a protein) in various ratios performed well. The PABR seemed to be minimally affected during the gradual substitution of glucose by gelatin. In fact, the reactor performance remained at an optimal level (approximate to98%), while operated under an organic loading rate of 3.125 g COD/l/d. (C) 2003 Elsevier Ltd. All rights reserved.