Beech wood derived xylan to hydrolyzed to predominantly xylose monomer units after exposure to hot, compressed liquid water saturated with carbon dioxide. Similar treatment without CO2 saturation resulted in only minor hydrolysis and a smaller fraction of monomers among the hydrolysis products Severity of the hydrolysis reaction was correlated to reaction time, temperature, and carbon dioxide partial pressure and followed a function similar to those used to characterize mineral acid systems. Results from parallel hydrolysis experiments with an aqueous system and a very dilute sulfuric acid system allowed an approximation of the dissociation constant of carbonic acid in the temperature range of 170-230 degreesC. Results suggest that carbonic acid may be a viable reagent for promoting hydrolysis without mineral acids, especially in the case of a bioprocessing plant that produces carbon dioxide.