Theoretical considerations on diffusion modes of adsorbates in diffusion-cell adsorbers are being investigated. By studying the effects of the operating and model parameter; on the response curves calculated by surface diffusion model and pore diffusion model, and noting the differences in the results, the following conditions are recommended for the prediction of the prevailing diffusion mode in diffusion-cell experiments lambda less than or equal to 0.1, Bi-B greater than or equal to 100, and N greater than or equal to 1 The theoretical prediction thus obtained checks well with experimental data taken from literature. New solutions are also presented for the surface diffusion model and the pore diffusion model with rectangular adsorption isotherm.