A pore diffusion model for the operation of a batch adsorber with external film diffusion resistance and adsorption isotherm is formulated. The analytical solutions for three geometric adsorbents: a slab, a cylinder, and a sphere, with linear adsorption isotherm can be obtained but they converge too slowly to be of much use numerically when the partition ratio is large and/or the Blot number is large. A new two-stage model, which is a close approximation, can give good agreement with the analytical solution throughout the time domain for wide range of partition ratio and Blot number. By comparing with the analytical solution, it is proved that the new two-stage model is much better than the parabolic two-stage model and the parabolic profile approximation. The influences of partition ratio and Blot number on the operation of a batch adsorber are also studied.