Reversible storage of hydrogen in microporous active carbon was performed by galvanostatic electrodecomposition of water either in 6 M KOH or in 1 M H2SO4 electrolytic solution. The most indicative proof of hydrogen insertion into carbon is a significant decrease (from 700 to 1000 mV) of the rest potential of the carbon electrode after the charge process, as well as a distinct discharge plateau during the oxidation process. Capacity of the stored hydrogen was estimated from the Faradaic charge passed during hydrogen oxidation. The uptake of ca. 2 wt.% reached in the alkaline medium is at least four times higher than the values obtained for the same material under pressure conditions, This enhancement is interpreted by the electrolytic formation of nascent hydrogen that penetrates easily in the useful porosity of the carbon nanostructure. (C) 2002 Elsevier Science B.V. All rights reserved.