Dimethyl phosphate anion (DMP Ϫ) is often used to simulate the properties of the phosphate group, an important structural fragment of nucleic acids and phospholipids. Besides its functional importance, DMP Ϫ has a strong affinity for metal cations and is often considered to be their binding site. In this study we reevaluate the binding properties of DMP Ϫ with water molecules and with explicitly hydrated magnesium and calcium cations in the presence of a polar continuum solvent. We analyze the stability of the DMP Ϫ in different conformations that represent possible inner-and outer-sphere binding modes. Our previous work (Petrov et al. J Phys Chem B 2004) has shown that electrostatic calculations, particularly those using a point-charge model, are wholly inadequate for this purpose. Calculations were performed at the B3LYP/6-31G(d,p) level of theory within the framework of the COSMO polarized continuum model (CPCM). Our data indicate that magnesium prefers outer-sphere coordination with DMP Ϫ , whereas calcium binds directly. The effect of cation binding on vibrational infrared and Raman spectra are also discussed. We found that the inclusion of polar solvent in the computations improves the agreement between calculated spectral data and experimental results.