Hemicellulose O-acetyl-galactoglucomannan (GGM) is the main noncellulosic water-soluble polysaccharidein the coniferous softwood Norway spruce, consisting of anhydro-galactose, -glucose, and -mannose. Acid hydrolysis of GGM has been studied in a continuous tube reactor to obtain these sugars under industrially relevant conditions. The reaction was performed under atmospheric pressure at 90°C and 95°C, and hydrochloric acid (HCl) served as catalyst. The influence of the reaction parameters, such as acid concentration (pH), temperature, concentration of the substrate, as well as catalyst and reactant flow rates, has been studied on the conversion efficiency and product distribution. Continuous production of monomeric sugars was achieved without formation of low-molecular by-products. The GGM conversion was high with HCl as catalyst, at 95°C, and a pH of 0.3. The main hydrolysis products were mannose, glucose, and galactose monomers. Minor amounts of sugar dimers were detected among the products. The experimental results are described with a laminar flow model for the continuous reactor.