Wood, the most abundant renewable raw materials on earth, primarily consists of cellulose, hemicelluloses and lignin with minor amounts of extractives and ash. The percentages of these components vary depending on wood species. The objective during chemical wood pulping is to retain the polysaccharides while removing the lignin. The Kraft process is presently the most widely used process in the paper industry. Approximately 20% of the dry weight of the wood is consumed during Kraft pulping process as hemicelluloses. The degraded hemicelluloses in the waste pulping liquor are then combusted during the Kraft recovery process. However, the heating value of wood carbohydrates, about 13.6MJ/kg, is only about half that of lignin. Therefore, a more economical use of the hemicelluloses would be to extract them prior to pulping, and then convert them to higher value-added products such as ethanol, itaconic acid and other chemicals. In order to maintain a high pulp yield as well as the integrity of the cellulose in the pulp, a fundamental understanding of hemicellulose dissolution during the extractive treatment is required. A modified Dionex ASE-100 (Accelerated Solvent Extraction Equipment) was utilized at elevated pressures in order to extract the hemicelluloses from mixed southern hardwood chips. The hardwood mixture was subjected to pre-extraction with water at 130 - 170 °C during 100 minutes extraction time using a modified Dionex ASE-100 reactor. The content of cellulose, hemicelluloses, lignin, acetyl groups and 4-O-methyl-glucuronic anhydride (4-O-MGA) that were retained in the extracted wood were determined following standard procedures. The extract was analyzed for oligosaccharides and monosaccharides, lignin, acetic acid, 4-OMGA, furfural and hydroxymethyfurfural (HMF). The extraction yield increased with increasing temperature. Xylan was the predominant component in the extract, especially at temperatures higher than 150 °C. The xylan dissolution increased exponentially with increasing temperature. Although xylan dissolved mainly as oligosaccharide, xylooligosaccharide were hydrolyzed to xylose at high temperatures (>160 °C). No significant amount of dehydration products, namely furfural and HMF, were generated at temperatures lower than 150 °C. However at temperatures higher then 150 °C small amount of both furfural and HMF were generated. Cellulose degradation was not significant during dissolution. Under the maximum conditions approximately 1% of the cellulose (based on wood) was dissolved in the water extraction in polymeric form. Arabinan and galactan are completely removed from the wood at 160 °C. Above 150°C, the component sugars such as arabinose, galactose, glucose and xylose in the extract were further degraded indicating complete hydrolysis had taken place with additional degradation. The residual xylan remaining in the wood was highly acetylated. The ratio of acetyl groups to xylose in the residual wood after pre-extraction with hot water was as high as that present in the original wood regardless of temperature. The ratio of 4-OMGA to xylose in the remaining wood decreased significantly with increasing temperature above 140 °C. This indicates that xylan was dissolved in acetylated form at all temperatures studied. In the extract, the ratio of acetyl groups bound to oligosaccharides to xylose decreased with increasing temperature, leading to a minimum ratio at approximately 155 °C. The ratio of 4-OMGA bound to oligosaccharides in the extract to xylose was almost constant. The final pH of the extract after pre-extraction at 100 minutes varied between 3.7 and 5.2 over the temperature range 130 to 170 °C.