The transient response of the enriching section of a packed distillation column to a step change in the reflux ratio was investigated. The solution in the case of a straight-line equilibrium curve was adapted to the real case ( w i t h a nonlinear equilibrium curve). This solution gives the liquid concentration, x, at any height of column, z, after the step change in the reflux ratio, in the form of a series of exponential functions of the time, f :where the S,'S are negative constants which depend only on the parameters characteristic of the final steady state, but not on the magnitude of the step disturbance, and.the coefficients C,'s depend on z only. A series of exponential terms is a convenient form of the solution for computing transfer functions,