The synthesis of the branched-macrocyclic ligand 1 incorporating two 2,2’-bipyridine units in the macrocycle and two 9-methyl-1,10-phenanthroline units in the branches is described as well as the synthesis and the photophysical properties of its Eu3+, Tb3+, and Gd3+ complexes. These complexes do not decompose in water in contrast to those of the related ligand containing 2,2’-bipyridine instead of 1,10-phenanthroline. They show intense absorption bands in the UV region due to absorption in the ligand. The emission spectra of the [Eu subset-of 1]3+ and [Tb subset-of 1]3+ complexes obtained upon ligand excitation show the usual Eu3+ and Tb3+ transitions. The pattern of the emission spectrum of the [Eu subset-of 1]3+ CoMpleX allows us to assess a low (presumably C2) symmetry as the probable site symmetry of the metal ion in the complex. For [Eu subset-of 1]3+ and [Tb subset-of 1]3+, the metal luminescence excitation spectra in water match the ligand absorption spectra while in methanol the absorption due to the phenanthroline is missing. This suggests that in water the efficiency of the ligand-to-metal energy transfer is similar for the two chromophores while in methanol phenanthroline transfers energy to the metal ion less efficiently than bipyridine. The luminescence quantum yield values in water and methanol confirm this interpretation. The lifetimes of the Eu3+ and Tb3+ emitting states indicate that the shielding of the metal ion from solvent molecules is rather inefficient. For the [Tb subset-of 1]3+ complex the lifetimes are temperature dependent which is attributed to the presence of an equilibrium between the metal emitting state and triplet excited states of the ligand; this process is most likely responsible for the low luminescence quantum yields and the oxygen effect on the Tb3+ luminescence. A detailed comparison between the photophysical properties in water and methanol allows us to conclude that the ligand 1 coordinates better to the metal ion in water than in methanol because of a stronger interaction in water between the phenanthroline branches and the metal ion. As to the application in fluoroimmunoassay, the Eu3+ and Tb3+ complexes of the ligand 1 present the highest molar extinction coefficients among the water-stable lanthanide complexes studied up to now. The value of the incident light-emitted light conversion efficiency, obtained from the absorption and emission efficiencies, makes the [Eu subset-of 1]3+ CoMplex interesting as a luminescent label.