The HF and DFT levels of theory were applied to study the interaction between monovalent cations and guanine tetrads. The calculations reveal that cation-guanine-tetrad complexes adopt the normal four-stranded Hoogsteen-bonded G-tetrad structure, and no bifurcated hydrogen bonds which stabilize the noninteracting G-tetrads were found. The gas-phase binding sequence between the monovalent cations and the G-tetrad complexes follows the order Li+ > Na+ > K+. After the hydration correction, the stability sequence of the monovalent cation-guanine-tetrad complexes in aqueous solutions follows the trend K+ > Na+ > Li+. The preferential binding of potassium over sodium and sodium over lithium in water solutions reproduces the experimental ion selectivity of the guanine tetraplex. In addition, the weak stabilization energy of the K+-G-tetrad in the coplanar form is consistent with the fact that the potassium cation tends to locate between two successive tetrads. The results of this study justify the conclusion of Hud et al. that the ion selectivity exhibited by the guanine tetraplexes in water solutions is dominated by the relative free energies of hydration. While the experimental measurement of the cation-oxygen distances in the sodium ion complex that is coplanar with the guanine tetrad in the crystal of a parallel-stranded guanine tetraplex (2.34 +/- 0.02 Angstrom) has been successfully reproduced at the HF level (2.331 Angstrom), the slightly shorter Na+-O6 distance and the significant decrease in the hydrogen bond lengths predicted by the DFT approach suggests an overestimation of the hydrogen bonding in the guanine tetrad by this method.