This work is part of a larger project whose main objective is a better understanding of the mechanism of adhesion between the surface of carbon fibres and resins in composites. The effect over the surface of commercial PAN-based carbon fibres (Courtaulds IM CG43-750) induced by several degrees of a commercial wet oxidative surface treatment (STL) as well as the nature of the fibre-size interface have been studied by X-ray photoelectron spectroscopy. As far as functional groups are concerned, a good qualitative and quantitative characterization of the fibre surface has been achieved, using a criterium based on the requirement of internal consistency between results obtained with photoelectrons from C 1s, O 1s and N 1s peaks. Oxidative treatment induces an enrichment in oxygen and nitrogen at the surface; the presence of the former on the surface is due to the treatment but strong evidence for a residual origin of nitrogen is obtained. Coating of the commercial fibres by a prepolymer similar to the resin used as the matrix, has been used to test the type of bonding between fibre and resin before curing. Evidence is given supporting the idea that the coating (size) does not entirely cover the surface of the fibre and that only a part of it is covalently bound, alcohol groups seeming to play an important role in the adhesion mechanism.