A rapid analytical methodology is proposed to answer the two questions about the molecular and structural features of the acidic xylo-oligosaccharides (XOSs) formed upon the enzymatic hydrolysis of 4-O-methylglucuronoxylan. The shortest acidic XOSs carrying a methylglucuronic acid moiety and the possible distribution of larger products (molecular feature) are instantly found by electrospray ionization mass spectrometry (ESI-MS) in the negative ion mode, which filters the unwanted neutral XOS. The acidic moiety is then unambiguously localized along the xylose backbone (structural feature) by ESI-MS in the negative ion mode via the selection/activation/dissociation of the product ions formed upon the one-way and stepwise glycosidic bond cleavage at the reducing end. Using the shortest acidic XOS with a known shape generated by glycoside hydrolase family (GH) 10 and GH11 xylanases as a proof of principle, pairs of diagnostic ions are proposed to instantly interpret the MS fingerprints and localize the acidic moiety along the xylose chain of the activated ion. The original structure of the acidic XOS is then reconstructed by adding as many xylose units at the reducing end as MS steps. Relying on pairs of ions, the methodology is robust enough to highlight the presence of isomeric products. Mass spectra reported in the present article will be conveniently used as reference data for the forthcoming analysis of acidic XOS generated by new classes of enzymes using this multistage mass spectrometry methodology.