This article is based upon the findings of our recent publication dealing with the liquid hot water (LHW) hydrolysis of lignocellulosic materials (LCM) in different reactor types [T. Rogalinski, T. Ingram, G. Brunner, Hydrolysis of lignocellulosic biomass in water under elevated temperatures and pressures, J. Supercrit. Fluids (2008), doi:10.1016/j.supflu.2008.05.003.]. As an advancement of these results, semi-continuous fixed-bed reactors were constructed aiming at the efficient hydrolysis of rye straw at elevated temperatures as a pretreatment process for bioethanol production. The temperature range between 170 °C and 230 °C was found to be most suitable for the thermal pretreatment of rye straw. The resulting liquid hydrolyzates as well as the solid residues were enzymatically converted to monomeric sugars (xylose/glucose) using cellulases and xylanases from Penicillium janthinellum. Ninety-five percentage of the initial xylan and 92% of the glucan was converted using a pretreatment temperature of 200–215 °C. The solubilization of the biomass in the fixed-bed reactor could be described by a reaction model following first-order kinetics. For that reason, the λmax-factor was introduced, which describes the maximum ratio of biomass that can be hydrolyzed at a certain temperature, related to the initial mass. The usage of a fixed-bed reactor combines several advantages compared to other reactor types, namely high solid-to-water-ratios, the prevention of degradation product formation, and considerable energy savings since no biomass comminution is necessary.