The electrochemical intercalation of sodium ions into pitch carbon fibres and natural graphite UF4 was studied using an electrolyte composed of ethylene carbonate as the solvent and NaClO 4 as the salt. The reversible electrochemical capacity for a current equal to 7 μA mg - 1 is negligible for graphite, a little bit higher for P100 fibres (28 mAh g - 1 ) and reaches 55 mAh g - 1 for P75 fibres. The sodium uptake can be increased by grinding our materials: the reversible capacity is then around 60 and 83 mAh g - 1 for ground P100 and P75 carbon fibres, respectively. During the first reduction, a great part of the current is used for the formation of a passivating layer on the carbon surface. Analysis of this layer by transmission electron microscopy (TEM) has shown that it is composed of sodium carbonate Na 2 CO 3 and alkylcarbonates ROCO 2 Na. Formation of Na 2 CO 3 occurs at potentials in the 1-0.8 V range versus Na + /Na, and formation of sodium alkylcarbonates then follows at potentials below 0.8 V. TEM also revealed the presence of sodium chloride, attributed to NaClO 4 reduction.