The problem of equilibrium and kinetics for adsorption-desorption of condensable vapors in porous media is studied experimentally and theoretically. For adsorption, the network model for diffusion based on por-e blocking theory, with percolation (in rite network) added by effective medium approximation is further introduced. A new predictive model based on properties of the Bethe lattices is proposed to account for the existence of liquid-filled "blind" pores that result in a decrease irt the total diffusion rate. For desorption, a new "shell and core"(or shrinking core) representation of the network model is proposed. Information from adsorption-desorption equilibria is needed to compute the thickness of the shell in which desorption/evaporation occurs for concentrations higher than the percolation threshold. These models form a unified equilibrium-kinetics theory for gas-porous solid systems that exhibit hysteresis. The models are applied to the systems silica gel-water vapor and Vycor glass-nitrogen. Concentration-dependent Fickian diffusivities for these systems have been measured for both adsorption and desorption branches. The adsorption model successfully predicts the experimental data with a maximum in diffusivity. The desorption model correctly predicts the concentration dependence of diffusivity with a steep minimum at the percolation threshold.