The most intriguing result of this study was the low strength of Ko-Z alk E: its zero-span tensile index value was remarkably below those reported for other degraded Ko samples (Figures and). Looking back at its MMD presenting a large fraction of relatively intact cellulose (Figure ), the low fibre strength can be regarded as an anomaly as it is contradicting a previous theory: the presence of a high-Mr cellulose fraction under the bimodal MMD of cellulose in pulp would be associated with retained mechanical properties despite an average DP loss . It is plausible that it is actually the location of this high-M r cellulose fraction within the fibre that matters. As detailed before, the cellulose least depolymerised by Z alk E treatment might be close to lignin fragments in Ko, ie. distributed along the cell wall layers. S2, the layer responsible for the high tensile strength of fibres with its low microfibrillar angle, is the thickest layer in wood fibres . For this reason we assume that contrarily to Ko-ZEo pulps being more damaged on the fibre surface, a substantial part of the S2 cellulose of Ko-Z alk (1.3)E has been depolymerised by radicals so it could explain the bad mechanical performance of this sample. Looking back to the first figure presented in this article, Ko-ZEo degradation type would be embodied by the crossed green dots concentrated in the outer parts of the fibre with limited consequence on fibre strength (Figure ). Conversely, purple dots more evenly distributed in the cell wall layers would illustrate the Ko-Z alk E degradation and is associated with a decrease of the zero-span tensile index due to the major role of S2 layer on tensile strength. These interpretations cannot be fully demonstrated with SEC and would require complementary analyses. Nano-indentation measurements on fibres cross-sections could be relevant in that case ).