In this research paper, a comparison between thermophilic and mesophilic anaerobic degradation of a variety of the simple sugar components of carbohydrate rich biomass is presented. In order to investigate the degradability of these basic sugars, three synthetic sugar based influents were supplied to two high rate upflow anaerobic hybrid reactors (UAHR) operated at 37 degrees C (R1) and 55 degrees C (R2). These influent streams were: D-glucose/sucrose: L-arabinose/D-xylose and L-rhamnose/D-galacturonic acid. The reactors were challenged in terms of influent composition rather than loading rate and were therefore operated at a maximum volumetric loading rate (VLR) of 4.5 gCOD l(-1) d(-1) during stable reactor performance. It was found that a switch from a D-glucose/sucrose synthetic influent to an influent composed of L-arabinose/D-xylose resulted in failure of the mesophilic reactor while the thermophilic UAHR was able to tolerate the change of sugar influent at an unchanged VLR of 4.5 gCOD A subsequent phasing-in approach was used to introduce new sugar influent streams and proved highly successful. The physiology of the biomass was assessed and it was noted that thermophilic anaerobic digestion (AD) involved the formation of acetate and H(2), implying the involvement of homoacetogenic bacteria, while mesophilic AD proceeded via the formation of other intermediates. (C) 2010 Elsevier Ltd. All rights reserved.