From the perspective of bio-refinery, sulfurous acid (H2SO3) treatment of lignocellulosic biomass is attractive because of its ability to act both as an acid catalyst and as a sulfonation agent. Therefore, its capability to hydrolyze polysaccharides (especially glucan) into monosaccharides was compared with two other acids, hydrochloric and sulfuric acids. The decomposition of methyl α-D-glucopyranoside (MGPα) in these three acids, hydrochloric, sulfuric, and sulfurous acids were studied. In addition, p-creosol and vanillyl alcohol were introduced to check whether it is possible to convert polysaccharides (such as hemicelluloses) into monosaccharides during the sulfurous acid treatment. The results showed that the decomposition of MGPα is much slower in H2SO3 than in HCl and H2SO4. The ligninsulfonic acid resulting from the lignin sulfonation reaction can be expected to improve the efficiency of hydrolysis of polysaccharides into monosaccharides during sulfurous acid treatment. Moreover, a higher actual yield of glucose was obtained in this case than in the other two acids.