Degradation of fermentable monosaccharides is one of the primary concerns for acid prehydrolysis of lignocellulosic biomass. Recently, in our research on degradation of pure monosaccharides in aqueous SO2 solution by gas chromatography (GC) analysis, we found that detected yield was not actual yield of each monosaccharide due to the existence of sugar-bisulfite adducts, and a new method was developed by ourselves which led to accurate detection of recovery yield of each monosaccharide in aqueous SO2 solution by GC analysis. By the use of this method, degradation of each monosaccharide in aqueous SO2 was investigated and results showed that sugar-bisulfite adducts have different inhibiting effect on degradation of each monosaccharide in aqueous SO2 because of their different stability. In addition, NMR testing also demonstrated possible existence of reaction between conjugated based HSO3- and aldehyde group of sugars in acid system.