It is proven that, given a constant flow rate throughout the system, the (2n) possible arrangements (including 180° rotation of any column along the long axis) ofnchromatography columns arranged in tandem yield identical elution curves (and therefore identical values of mean residence time and variance). The columns must be of the same cross-sectional area but each can be of arbitrary length. The proof is independent of the nature of the packing material in each column and holds for any given input distribution in time and/or constitution.