•Different wood particle sizes were studied for hemicellulose extraction at 170°C.•Smaller particle sizes gives faster extraction and slightly better yield.•The reaction order was 1.5st for 0.5–2mm and pseudo-first for 2–12.5mm particles.•Hemicellulose molar-mass dropped rapidly during the first 10min of extraction.•For maximum yield of high-molar-mass hemicelluloses 20min extraction was optimal. The effect of five different wood particle size fractions between 0.5 and 12.5mm on hot-water extraction of acetylated water-soluble hemicelluloses from spruce wood with a batch extraction setup at 170°C was investigated. Extraction kinetics, with regard to particle size, was also studied. The purpose was to intensify the hemicellulose extraction for high molar mass hemicelluloses at high yield and purity. About 30% of the wood was dissolved and basically all the hemicelluloses could be extracted. The average molar masses of the extracted hemicelluloses decreased rapidly during the first 10min of the extraction, but were not much affected by the difference in wood particle sizes. Smaller particles resulted in higher extraction rates. The reaction order was established to be of pseudo-first order for particles above 2mm and 1.5st order for particles smaller than 2mm. The effective diffusion coefficient was determined to be 9.11×10−10m2/s.