Abstract A new method was developed for determining the stage efficiency of a liquid—liquid extractor. The method is based on solute injections to input streams of the extractor operating with a total recirculation of phases. The procedure does not require large amounts of liquids. No further knowledge of flow structure is needed; the extractor is considered as a “black box”. The representation of the extraction unit by the transition matrix of Laplace transform functions and its limiting properties forms a theoretical background for the method. The evaluation of experiments does not require any complicated computations. In a set of experiments illustrating the proposed method remarkable dependencies of the efficiency on the hold-up and the type of dispersion were observed.