Adsorbed natural gas (ANG) is an interesting opportunity for developing natural gas vehicles technology. In this case, adsorbents such as activated carbons are used to store natural gas at moderate pressure, 3.5 MPa, compared to the high-pressure (20 MPa) required for current compressed natural gas technology. Many studies are devoted to the elaboration of suitable adsorbent materials to optimise the methane storage capacity. Nevertheless, since natural gas (NG) is composed of about 95% of methane mixed with other components, an important deterioration of the storage performance is observed after successive cycles of an ANG system (filling and delivery). It is the result of adsorption of the other components present in NG that are mainly higher molecular-weight hydrocarbons, carbon dioxide and nitrogen. In the present study, the evolution of the storage capacity of a 21 vessel cycled with NG is experimentally measured. As a function of the cycle number, the gas composition at the outlet of the ANG system is determined using gas chromatography analysis. (C) 2004 Elsevier B.V. All rights reserved.