AbstractHot water extraction of hemicelluloses from aspen wood chips of different sizes and wood meal was performed, and the extracted hemicelluloses were characterized. With decreasing wood chip size, the total sugars and total poly- and oligosaccharides extracted with hot water increased. The dissolution of hemicelluloses was mainly limited by diffusion in the fiber wall and mass transfer from fiber to fiber during hot water extraction of the wood chips. The extraction yield of high molecular weight hemicelluloses was less than that of low molecular weight hemicelluloses, and the reduction of wood chip size benefited the extraction of high-molecular weight hemicelluloses. Compared to the wood chips, the high molecular weight hemicelluloses extracted from wood meal had a higher extraction yield and an increased average molecular weight.