In measurements of diffusion coefficients of gases in porous solids by unsteady-state (transient) methods, adsorption on the walls of the pores affects the analysis. Failure to account for the net adsorption or desorption occurring during the perturbation associated with such measurements yields apparent diffusion coefficients that are lower than the true values. Moreover, the temperature dependence is not the relatively weak dependence expected from the kinetic theory of gases for true gas diffusion coefficients, even when the pores are orders of magnitude larger than molecular dimensions. Instead it is exponential in nature, being determined largely by the temperature dependence of the adsorption process occurring in the pores. Thus, the diffusion process seems to be an activated one, although in fact there may not be a true activation barrier. The analysis presented in this paper appears to be very general, extending even to zeolites where the pore dimensions are of the order of molecular dimensions. It appears to account for the commonly reported observations in the Literature that diffusion coefficients for gases in microporous solids obtained by transient methods can sometimes be as much as two to four orders of magnitude lower and can simultaneously exhibit temperature dependencies much stronger than those obtained under equilibrium conditions utilizing nuclear magnetic resonance.