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A mixture of US southern hardwood chips was treated with aqueous acetic and formic acid (FA) solutions in a modified Dionex ASE100 extractor. The amount and selectivity of hemicelluloses dissolution increase at higher acetic acid (AA) and FA concentrations, but cellulose remains intact in the residual wood. Even at the highest AA concentration of 100 g l-¹, the dissolved hemicellulose fragments are still oligomeric except for arabinose. The formation of lignin-like condensation products (LCPs) and decomposition of dissolved mannose, arabinose, and galactose were observed at the highest AA concentration. Sugar hydrolysis is more pronounced with the addition of FA at increasing temperature. The concentration of xylose in the FA extract is approximately the same as that of xylooligomers at 160°C. Formation of LCPs is only observed at 170°C during 10 g l-¹ FA treatment. The concentrations of dissolved wood components increase linearly with increased recycling of the FA extract. [ABSTRACT FROM AUTHOR]