Two new tetraazamacrocyclic ligands are designed with the aim of sensitizing the luminescence of Tb(Ill) and Eu(Ill) ions in water: L5{1,4,7,10-tetrakis[N-(phenacyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane} and L6 {1,4,7,10-tetrakis[N-(4-phenylphenacyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane}. These ligands react with lanthanide trifluoromethanesulfonates to yield stable 1:1 complexes in water (log K = 12.89 +/- 0.15 for EuL5). X-ray diffraction on [Tb(L5)(H2O)](CF3SO3)(3) (P (1) over bar, a = 13,308(3) Angstrom, b = 14.338(3) Angstrom, c = 16.130(3) Angstrom, alpha = 101.37(3)degrees, beta = 96.16(3)degrees, gamma = 98.60(3)degrees) shows the Tb(III) ion lying on a C-4 axis and being 9-coordinate, with one water molecule bound in its inner coordination sphere. The absolute quantum yields are determined in aerated water for the complexes formed with ions used in fluoroimmunoassays (Ln = Sm, Eu, To, and Dy). Large values are found for [Tb(H2O)(L5)](3+) and [Eu(H2O)(L6)](3+), in line with the molecular design of the receptors: 23.1% and 24.7%, respectively. The intense luminescence of these ions results from efficient intersystem crossing and L Ln energy transfer processes, as well as from a suitable shielding of the emitting ions from radiationless deactivation.