A high temperature in-line pH control system was used during hot-water extraction of spruce wood for enhancing the extraction towards high yield of water-soluble high-molar-mass hemicelluloses. Extrac-tions of 2-4 mm wood particles and industrial size wood chips at 180°C and pH set points 5.15, 4.85, and 4.50 were performed, as well as reference extractions with no pH control. The extracts were analyzed for total dissolved solids, extracted hemicelluloses and sugar composition, monomeric sugars, molar mass of the extracted hemi-celluloses, and released acetic acid. Results show that higher extraction pH decreases the yield of extracted hemicelluloses, but increases their average molar mass. This active pH control is to be preferred against other, passive pH control methods. The main advantages are better monitoring and control of the actual extraction pH, only the needed chemicals are added, and of course a stable extraction pH of choice.