When glassy carbon fibres are used, alkali and alkaline earth metal ions are adsorbed in the micropores which are created or activated by oxidative treatment of the fibres. The average pore radius was estimated to be 2 nm by nitrogen adsorption experiments. Uptake was studied with a flow system consisting of a flow-through carbon-fibre electrode; a dropping mercury electrode at the end of the flow line monitored changes in the metal ion concentrations. The ions were taken up at negative potentials and released at positive potentials. The amount taken up increased in the sequence Na + < Li + < H +, suggesting that the ions were desolvated before entering the pore system.