Linking these visual degradation patterns to fibre strength evaluated by zero-span tensile measurements further corroborate some conclusions from the past. Ozone and hypochlorous acid treatments which affect almost indifferently the whole of the fibre have less impact on fibre strength than localised attacks like those of cellulase enzymes, even though the DP can drop significantly. The superior level of details given by SEC over simple pulp viscosity measurements proves that both ozone and hypochlorous acid leave a bimodal cellulose fraction, with some cellulose being relatively intact. This fraction had hitherto been thought responsible for maintaining fibre strength despite the DP loss. Our new results include an unconventional ozone treatment performed in alkaline conditions and resulting a substantial fibre strength loss. Consequently, we believe that it is the degree of degradation of the fibre core (S2 layer) that would rule kraft fibre strength deterioration through various chemical treatments. This could explain why conventional ozone bleaching, by depolymerising cellulose mostly located in the fibres surface, does not cause substantial strength loss.