Responsive lanthanide(III) complexes—europium(III) complexes in particular—has been the focus of studies for several decades. The response, in the form of changes of photophysical and electronic properties of the lanthanide(III) ion, arises through supramolecular interactions between a guest or analyte molecule, and the lanthanide(III) complex which acts as host. While responsive lanthanide complexes have been reported and investigated frequently, the supramolecular aspects and linear free-energy relationship have had less attention. Here, we are revising five europium(III) complexes and investigating their binding to nine different guests, all with a primary interaction between europium(III) ion and a bidentate carboxylate anion. The media effect was investigated, and by eliminating the impact of hydrophobic effects, we can show that selectivity in these host-guest systems can be tuned by the secondary lipophilic interactions.