Nuclear magnetic resonance (NMR) spectroscopy shows that > 50% of the carbon in marine high molecular weight dissolved organic matter (HMWDOM) from surface waters is a compositionally well-defined family of acylated polysaccharides that is conserved across ocean basins. However, acid hydrolysis of HMWDOM, followed by chromatographic analysis, recovers only 10-20% of the carbon as neutral, amino and acidic sugars. Most carbohydrate in HMWDOM therefore remains uncharacterized. Here, we use acid hydrolysis followed by Ag + and Pb 2+ cation exchange chromatography to separate HMWDOM hydrolysis products for characterization using 1-D and 2-D NMR spectroscopy. In addition to neutral sugars identified in past studies, we find 3-O-methylglucose, 3-O-methylrhamnose, 2-O-methylrhamnose and 2-O-methylfucose. We also find 3-deoxysugars to be present, although their complete structures could not be determined. Methyl sugars are widely distributed in plant and bacterial structural carbohydrates, such as cell wall polysaccharides, and their presence in HMWDOM suggests that structural carbohydrates may contribute to DOM in surface seawater. We find most HMWDOM carbohydrate is not depolymerized by acid hydrolysis and that the non-hydrolyzable component includes 6-deoxysugars.