The virial analysis of the adsorption isotherms of methane on activated microporous chars has been used to investigate the behavior of the gas-solid interface at the temperature T = 298.2 K. A commercial char, CNS-201, is the raw material used in this study. Steam (vaporized water, H2O) has been used as the activating agent for the production of the samples tested. The two-dimensional gas-solid second virial coefficients, B-gs, deduced from experimental data are analyzed as a function of the duration of the activation process. We observed that the structural changes of the char during the activation process are described by the variation in the value of B-gs. The virial treatment is useful to investigate the molecular behavior related to physisorption and to characterize the evolution of microporosity during the activation.