Microbial endo-β-1,4-xylanases (EXs, EC 126.96.36.199) belonging to glycanase families 10 (formerly F) and 11 (formerly G) differ in their action on 4-O-methyl-d-glucurono-d-xylan and rhodymenan, a β-1,3-β-1,4-xylan. Two high molecular mass EXs (family 10), the Cryptococcus albidus EX and X1nA of Streptomyces lividans, liberate from glucuronoxylan aldotetrauronic acid as the shortest acidic fragment, and from rhodymenan an isomeric xylotriose of the structure Xylβ1-3Xylβ1-4Xyl as the shortest fragment containing a β-1,3-linkage. Low molecular mass EXs (family 11), such as the Trichoderma reesei enzymes and XlnB and XlnC of S. lividans, liberate from glucuronoxylan an aldopentauronic acid as the shortest fragment, and from rhodymenan an isomeric xylotetraose as the shortest fragment containing a β-1,3-linkage. The structure of the oligosaccharides was established by: NMR spectroscopy, mass spectrometry of per-O-methylated compounds and enzymic hydrolysis by β-xylosidase and EX, followed by analysis of products by chromatography. The structures of the fragments define in the polysaccharides the linkages attacked and non-attacked by the enzymes. EXs of family 10 require a lower number of unsubstituted consecutive β-1,4-xylopyranosyl units in the main chain and a lower number of consecutive β-1,4-xylopyranosyl linkages in rhodymenan than EXs of family 11. These results, together with a greater catalytic versatility of EXs of family 10, suggest that EXs of family 10 have substrate binding sites smaller than those of EXs of family 11. This suggestion is in agreement with the finding that EXs of family 10 show higher affinity for shorter linear β-1,4-xylooligosaccharides than EXs of family 11. The results are discussed with relevant literature data to understand better the structure-function relationship in this group of glycanases.