Natural gas is a good fuel for internal combustion engines, but its low energy density is a significant drawback. The energy density can be increased by adsorption on a high surface area activated carbon. But with usage, some of the constituents in the natural gas composition accumulate on the carbon and reduce its adsorptivity. The adsorption-desorption of natural gas on 9LXC activated carbon was investigated to 100 cycles at 21 °C and pressures of up to 12 MPa. The decrease in the capacity, G, as a function of the number of cycles, N, was found to follow the empirical correlation:G/Go = 1 − 0.085Log (N). Analysis of the activated carbon after 100 cycles showed accumulation of C4. and higher hydrocarbons but not of C2 and C3. For automotive applications, activated carbon appears practical in a narrow pressure range, centering around 7 MPa (1000 psig). The preferred storage is at a pressure of 17 MPa or higher, without the use of activated carbons.