Monovalent cations play an important role in many biological functions. The guanine rich sequence, d(G(4)T(4)G(4)), requires monovalent cations for formation of the G-quadruplex, d(G(4)T(4)G(4))(2). This requirement can be satisfied by thallium (Tl+), a potassium (K+) surrogate. To verify that the structure of d(G4T4G4)2 in the presence of Tl+ is similar to the K+-form of the G-quadruplex, the solution structure of the Tl+-form of d(G4T4G4)2 was determined. The 10 lowest energy structures have an all atom RMSD of 0.76 +/- 0.16 angstrom. Comparison of this structure to the identical G-quadruplex formed in the presence of K+ validates the isomorphous nature of Tl+ and K+. Using a H-1 -(TI)-T-205 spin-echo difference experiment we show that, in the Tl+-form of d(G(4)T(4)G(4))(2), small scalar couplings (< 1 Hz) exist between Tl-205 and protons in the G-quadruplex. These data comprise the first 1H -205Tl scalar couplings observed in a biological system and have the potential to provide important constraints for structure determination. These experiments can be applied to any system in which the substituted Tl+ cations are in slow exchange with the bulk ions in solution.