The proposed research is aimed at developing a theory for predicting the dynamic behavior of countercurrent mass-transfer columns. All previous models are aimed at predicting the steady-state behavior. The theoretical development will begin with a single component and with finite mass transfer rates to be followed by multicomponent mixtures. Experiments will be conducted on countercurrent packed-column extraction, with concentration disturbances. The effects of reflux on the column dynamics and on the stability will then be analyzed. The dynamic analysis results will be useful in process design and in the control of a host of countercurrent mass-transfer operations, such as distillation, adsorption, absorption, and extraction.