A model for transport and adsorption of gases in a biporous solid sphere was presented assuming a linear adsorption isotherm and linear concentration dependence of surface diffusivity. When put into dimensionless form this model is characterized by four dimensionless parameters. To help assessing the error involved in using constant effective diffusivities, numerical results were presented for the uptake of pure gases into a biporous solid sphere with concentration dependent diffusivity and compared with those for effective diffusivity model. The error in transient uptake was found to be as large as ten percent.