ENEA (Italian Agency for New Technology for the Energy and for the Environment) research on the anaerobic treatment of raw cheese whey started in 1990 with the objective of developing a technology suitable for medium-size cheese factories that have growing disposal problems and cannot afford high investment costs for whey valorisation technologies (such as whey protein and lactose recovery, spray drying, etc.). The first attempt was to run a mesophilic hybrid upflow reactor that was able to reach high removal performances but showed pH unstability which led to failure. In order to obtain better process control, experiments continued on a two-phase CSTR (completely stirred reactor) system that gave good results in terms of stability. To couple process stability and high loads, a new reactor called a downflow-upflow hybrid reactor (DUHR) has been designed. The reactor was able to reach B-v (volumetric organic loading rate) values around 10 000 mg COD l(-1) day(-1), with 98% COD removal and effluent soluble COD (CODsol) values close to 1000 mg.l(-1) no external addition of alkalinity was required to maintain a stable pH that was constantly around 6.5-6.7 in the downflow pre-acidification chamber and around 7.5 in the bio-methanation upflow chamber. Copyright (C) 1996 Elsevier Science Ltd.