This study investigates how partial modification of softwood Kraft fibres into dialcohol cellulose affects fibre wall integrity and swelling behaviour across a modification range of similar to 25- similar to 50%. Structural transformations within the fibre wall and the roles of the secondary cell wall layers in generating balloon-like morphologies during heterogeneous swelling were examined. A combination of optical microscopy techniques including polarised light, differential interference contrast and confocal laser scanning microscopy with dual fluorescent labelling was employed to visualise morphological changes. Results show that swelling intensified with increasing modification. At 51% modification, fibres exhibited uniform swelling, and the characteristic balloon-collar-like structures disappeared. Mild ballooning was observed in "never-dried" fibres with similar to 25% modification. Fibre width increased with modification, ranging from 35 +/- 9 mu m (unmodified) to 58 +/- 24 mu m (similar to 50% modification), with greater variability at higher modification levels. Water retention values also rose, from 1.7 to 6.3 g water per gram of fibre. Finally, the modification-induced swelling introduced inelastic strain in the fibre wall, preserving balloon-collar morphology in air-dried fibres.