An approximation formula for estimation of mass exchange rate between a biporous adsorbent and its surrounding that undergoes a cyclic change in concentration is theoretically developed. The formula is structurally simple and linear with two fully predicted coefficients, and it is valid for any cycle period. The formula is shown to reduce to the classical linear driving force (LDF) equation and to agree with previous approximations when the period of cyclic operation is larger than the diffusional time constant theta(p). In the practically important range of the cycle period shorter than theta(p), only the present formula is capable of accurately predicting the cyclic mass transfer in the biporous adsorbent. An example is provided to demonstrate that the present formula is a useful alternative to the complex bidisperse pore diffusion model, particularly in the simulation of eyelid sorptive separation processes.