The electrochemical insertion of sodium ions into pitch-based and PAN-based carbon fibres was studied in the liquid electrolyte composed of ethylene carbonate as the solvent and NaClO4 as the salt. The sodium uptake depends on the nature of the carbon precursor and on the manufacture mode. In pitch-based materials, the reversible electrochemical capacity is higher in the less graphitized structure up to a Na0.31C8 composition. In PAN-based fibres, sodium inserts massively to give the metal-rich Na0.75C8 compound. Two concomitant insertion mechanisms can be proposed: one consists in sodium intercalation between the graphene layers while the other is related to sodium adsorption in the material pores. The comparison of the electrochemical and the X-ray diffraction results indicates that the intercalation process is predominant for pitch-based fibres whereas the sorption phenomenon occurs mainly in the PAN-based structures. The contribution of each mechanism in relation with the characteristics of the fibres is discussed in order to select the host material presenting the best electrochemical properties.