A kinetic model for xylan hydrolysis in liquid hot water flow-through pretreatment was developed. The model utilized a declining xylan hydrolysis rate constant with increasing conversion in combination with direct xylooligomer degradation. The model was able to describe experimental results from flow-through pretreatment of corn stover and triticale straw at various pretreatment temperatures, and was applied to predict and compare the performance of xylan hydrolysis in co- and countercurrent flow-through pretreatments. Countercurrent pretreatment resulted in higher concentration of solubilized xylan and 36-fold less degradation. Maintaining a temperature gradient along the reactor axis reduced degradation compared to a fixed reactor temperature. Biomass bed shrinking during pretreatment increased the final concentration of solubilized xylan by about 10%. Model predictions were sensitive to the packing density of biomass bed. The model is useful for evaluating biomass flow-through pretreatments and has utility in design of flow-through reactors. (C) 2012 Elsevier Ltd. All rights reserved.