A mathematical description is presented for xylan solubilization during hydrothermal pretreatment of sugar maple and aspen wood chips. It assumes first-order kinetics with Arrhenius-type temperature dependence of the reaction rate constants and includes limitations on the transport of reaction products formed in the wood pores to the bulk liquor. The model is calibrated with experimentally measured yields of residual xylan, xylooligomers, and xylose obtained in the autohydrolysis of sugar maple wood chips under pretreatment conditions of 152-175 degrees C, reaction times of 0-6 h, and liquor to solid ratio of 6:1 in a batch digester. It is validated against experimental data obtained from the autohydrolysis of aspen wood chips at 160 degrees C under similar experimental conditions as those used for sugar maple. The model can also satisfactorily correlate data of deacetylation of xylan and formation of products (free acetyl groups and acetyl groups attached to xylooligomers in the hydrolyzate) in the autohydrolysis of sugar maple and aspen wood chips with fair accuracy. (C) 2009 Elsevier Ltd. All rights reserved.