The formation of binary and ternary model complexes of Eu(III) with the nucleobases 5-aminouracil (5-amino 2,4-dioxy pyrimidine), dihydrouracil (5,6-dihydro-2,4-dioxy pyrimidine), thymine (2,4-dihydroxy 5-methyl pyrimidine), adenine (6-aminopurine), uracil (2,4-dioxy pyrimidine), and PIPES (piperazine 1,4-bis (2-ethane sulfonic acid) dissodium salt) has been studied potentiometrically at (25.0 +/- 0.1)degrees C and at an ionic strength of I= 0.1 mol . dm(-3) (KNO3). The formation of the 1:1, 2:1 binary, and 1:1:1 and 2:1:1 ternary complexes is inferred from the corresponding titration curves. Initial estimates of the formation constants of the resulting species and the protonation constants of the different ligands used have been refined with the SUPERQUAD computer program. The experimental conditions were selected such that self-association of the nucleobases and their complexes was negligibly small; that is, the monomeric and protonated complexes were studied. Recognition of nucleobases and CT-DNA by the luminescent bioprobe Eu(III) PIPES has been carried out. The solid Eu(III) PIPES complex was synthesized and characterized using elemental analysis, mass spectra, and IR spectroscopy. The interaction of an aqueous solution of the Eu(III) PIPES complex with CT-DNA was examined using cyclic voltammetry (CV), differential pulse polarography (DPP), and square wave voltammetry (SWV). The fluorescence emission characteristic band for the Eu(III) PIPES complex is enhanced by the addition of various concentrations of CT-DNA.