Two novel GH11 endo-xylanases from Myceliophthora the rmophila Cl (Cl), Xy17 and Xy18, were purified and the influence of solubility and molecular structure of various xylans on their efficiency was investigated. Both endo-xylanases were hindered by a high degree of substitution of a xylan. The two GH11 xylanases released different products from the xylatis, in which Xy17 displayed a degradation product composition closer to GH10 xylanases. A correlation of the degradation product composition with a specific residue at position 163 in the amino acid sequence of Xy18 is suggested: tyrosine in Xy18; valine in Xy17. This is confirmed with examples of various endo-xylanases reported in literature. The C1 GH11 xylanases were more efficient on self-associated xylan compared to C1 GH10 endoxylanases and they released more small xylooligomers from these xylans. This is contrary to the general assumption that GH10 xylanases degrade xylans to a higher degree than GH11 xylanases. (C) 2013 Elsevier Inc. All rights reserved.