A novel complex of monomeric thallium(III) with the nitrogen donor ligand phenanthroline (phen) has been prepared and characterized by multinuclear NMR (H-1, C-13, Tl-205). The three complexes exist in equilibria in DMSO and acetonitrile solution which was proved by the Tl-205 NMR spectra. The H-1 and C-13 NMR spectra of tris-phen Tl(III) complex have been measured, where the spin-spin coupling between Tl (I = 1/2) and C-13 or H-1 signals were observed with the H-1 and C-13 NMR spectroscopy in acetonitrile. The coupling constants are presented and the chemical shifts of complexes are discussed in detail.