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A model is presented for studying the dynamic behavior of stagewise processes with non-ideal stages and back-mixing between them. By numerical integration on a computer, solutions have been obtained for two variants of the model: with a linear and non-linear equilibrium relationship respectively. For the first variant an approximate relation was obtained for calculating the start-up time. The analysis presented has been experimentally verified for a reciprocating-plate extractor.c dynamic behavior of mass transfer processes is attracting Th increasing interest. Papers published on this topic deal cither with problems associated with control of such processes or with the start-up time. The essential requirements on models for the first of thc two purposes named are that they yield a solution rapidly, and that they be sufficiently accurate especially for the initial period of the responsc. With models for the latter purpose, howcvcr, the time required for obtaining a solution is not a critical factor, but a sufficiently high degree of accuracy is required ovcr the entire start-up period. The second type of model can hc utilized also for studying the optimal start-up regime: either to attain steady state in the shortest time, or to minimize loss of product. These aspects arc of special significance in semi-continuous production lincs.Models developed for studying the dynamic behavior of extraction and absorption plant can be classified into the following groups: I . Distributed-parameter models (a) for differential contact of phases (I-@, (b) for differential contact of phases, but including the effect of axial dispersion @-Io). 2. 1,umped-parameter models (a) for stagewise contact (l*6-10*11), (h) for stagewise contact with non-ideal stages ( l 2 -I b ) , (c) for stagewise contact with non-ideal stages and back-