AbstractThe unsteady‐state behaviour of a reciprocating plate column in which adjacent plates move out of phase with each other has been studied experimentally by determining responses to step changes.The one‐dimensional, two‐parameter model developed, leading to a system of differential equations with time‐dependent coefficients, was simplified to allow parameter estimation in the Laplace domain. Special features of the experimental technique for obtaining instantaneous values of the tracer concentration which closely approximate to the average for a column cross section are described; the limitations of parameter estimation in the Laplace domain are di