An experimental method is illustrated, which opens up an effective line of attack in the fundamental and practically unexplored field of not-necessarily-dilute, not-necessarily-low-charge, multiple electrolytes. The method is based on the idea that ions of a sufficiently enhanced hydrophobic character (typically, charged metal complexes and organic or partially organic ions) can be studied by means of liquid membrane cells without undergoing any interference by highly hydrophilic ions such as SO₄²⁻, OH⁻, H⁺, Mg²⁺, La³⁺, etc. Proofs are given for hexacyanocobaltate(III) and tris(ethylenediamine) cobalt(III) salts. As a first application, the method is used to find otherwise unknown activity coefficients of trace electrolytes in swamping ionic bulks – the typical situation encountered in equilibrium, kinetic constant, and standard potential determinations. Observed salt effects do not prove in any case to be trivial, and in particular, the activity coefficients of 3:3 salts in 2:2 and 3:2 ionic bulks are found to be completely different from expectations.