By means of a model the effect of axial dispersion on mass transfer in countercurrent contactors is described. The mixing is represented by a cascade of a number of ideal mixers in each phase. The separation performance-characterized by the number of external transfer units, ∫ dC/(C* C)-is in a simple analytical way related to the extraction factor, mFx/Fy, to the number of internal transfer units, kox.a.L/vx, and to the number of ideal mixers in each phase. The number of external transfer units calculated with this model is in good agreement with the results of the longitudinal dispersion model as presented by Miyauchi and Vermeulen. The simplicity of the model enables rapid slide rule calculations and provides rapid recognition of the factors limiting the separation performance.