Pressurized hot water extraction of hemicelluloses from spruce sapwood was studied at 120-170 degrees C using a batchwise-operated cascade reactor, which enables precise sampling as well as very accurate and rapid temperature control. The extraction was performed under identical conditions for two different chip sizes, a 1.25-2.0-mm sieved fraction and handmade 10-mm cubic blocks, to evaluate the influence of chip size on the overall extraction kinetics. The results showed that the extraction rate increases significantly with temperature and that the pH decreases during the extraction, as a result of the liberation of acid groups. The concentration of hydronium ions in the liquid phase was observed to have a linear correlation with conversion depending, however, on the chip size, which shows that the mass transfer of the acid groups differs significantly from that of the bulky hemicelluloses. It also shows that significant amounts of acetyl groups are liberated inside the chips before the hemicelluloses enter the liquid phase, as the slopes would otherwise be identical. The extraction temperature did not influence the selectivity of dissolution significantly, which means that temperature cannot be used to influence the sugar composition of the obtained liquid phase. Mathematical modeling was performed on the overall extraction data using a simple first-order model, which corresponds to porous solid particles. An excellent fit of the model to the experimental data was obtained. The activation energy was determined to be about 120 kJ mol(-1) for both chip sizes even though the reaction rates differed significantly, wheeras the pre-exponential factor was substantially lower for the larger chips. This somewhat surprising result can be explained by the fact that the diffusion inside the chips differs because of changes in viscosity and not only distance. The results contribute to the quantitative and qualitative understanding of the extraction process and shed light on the correlation of the experimental parameters used during extraction.