Modifications of the morphological and chemical structures of polypropylene fibres, resulting from treatment with ozone, were examined by SEM and i.r. spectroscopy. The morphological study showed on the “ozonized” fibre, a well-defined peripheral zone (“skin”), with fragile-type mechanical behaviour: the thickness of this zone is near 2 μm, and it contains many radial cracks with fairly constant dimensions. That effect, which seems original, could be related with a peculiar organization of the skin structure: the skin might be either more crystalline than the core, or, conversely, more amorphous (and, in fact, in a vitrous state at liquid nitrogen temperature). Such a well-defined skin-core morphology is completely absent from the initial non-oxidized fibre. i.r. Spectroscopy confirmed that chemical structure modifications of the PP fibre, due to ozonization, are in good agreement with those described in the literature for similar situations: newly-formed functions are ethylenic or hydroxyl groups, carboxyls or ketones, esters or aldehydes, cyclic compounds with 4–5 carbons (γ-lactones), and H-bonded hydroxyl and hydroperoxide groups. Chemical reactions likely to be responsible for the production of such groups are proposed. Finally, it was shown that weight increase after zone treatment was directly linked to the production of the functional groups.