Responsive lanthanide(III) complexes – europium(III) complexes in particular – have been the focus of studies for several decades. While responsive lanthanide complexes have been reported and investigated frequently, the supramolecular aspects and linear free-energy relationship have seen less attention. Here, we are revisiting five europium(III) complexes and investigating their binding to ten different guests, all with a primary interaction between the europium(III) ion and a bidentate carboxylate anion. The studies consisted of measuring emission spectra, analysing band behaviours of the europium transitions, and calculating lifetime isotherms. The media effect was also 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.