The interface between the fibre and the matrix is a very important factor influencing the mechanical behaviour of composite materials. For superior composite performance, one must not only select optimal fibres and matrices, but also optimize the interface between them. However, the control of the interface properties is not an easy task. This work is an interdisciplinary and integrated approach to the problem. The effect of different degrees of a wet oxidative surface treatment on the surface of poly(acrylonitrile) based intermediate modulus carbon fibres (Courtaulds IM CG43-750) has been studied using classical thermodynamic as well as spectroscopic techniques, aimed at obtaining a complete physical and chemical characterization of the fibre surface. The results show that all aspects of the fibre surface are influenced by the surface treatments, which are specially designed to improve the adhesion between fibre and matrix. The study outlines the most important surface features im proving th is adhesion. The results concerning the characterization of the fibre surface contribute, when combined with micromechanical tests, to clarifying the adhesion mechanisms, revealing, at the same time, a mechanical interlocking and a chemical interaction.