•Selectively oxidized 2,3-dicarboxypolysaccharides has number of unique properties.•Direct control over their molecular weight is achieved by addition of sulfamic acid.•Underlying mechanism is elucidated for cellulose and dextrin.•Role of reaction conditions to molecular weight of products is investigated.•Materials with adjustable molecular weight and degree of oxidation >90% prepared. Oxidation of polysaccharides to 2,3-dicarboxypolysaccharides is a two-stage process, where selective oxidation by periodate is followed by secondary oxidation by chlorite. Addition of sulfamic acid before the secondary oxidation influences the molecular weight and degree of oxidation of the product. Here, mechanism of sulfamic acid-catalysed chain scission is elucidated for selectively oxidized cellulose and dextrin. Initially, sulfamic acid sulfonates the aldehyde groups of 2,3-dialdehydepolysaccharide. Introduced –SO3H groups are in ideal position to protonate the oxygen atom of 1-4′ glycosidic bond and to trigger acidic hydrolysis. This can be used to obtain a direct control over the molecular weight of the product. Observed slightly lower degree of oxidation was ascribed to the ability of sulfamic acid to scavenge the hypochlorite and thus protect the intermolecular hemiacetals from oxidation. Usually undesirable hypochlorite thus seems to be necessary for preparation of selectively oxidized polysaccharides with degree of oxidation above 90%.