The phosphate geometry was studied in crystal environment by analyzing 178 crystal structures and in vacuo by ab initio calculations of (di)hydrogen and dimethyl phosphates and a diphosphate model system, CH3OP(O-2)O(CH2)(3)OP(O-2)OCH3 (diPh), at the MP2 and HF levels. Since charge determines the phosphate stereochemistry, uncharged, -1, and -2 phosphates were analyzed separately. The P=O and O=P=O bonding parameters depend only on charge and the number of carbon substituents while the remaining parameters depend also on substituent type; the largest sensitivity was observed for the C-O and C-O-P parameters. An acceptable agreement between the crystal and ab initio geometries was obtained only when basis sets contained polarization functions and a model system included a counterion (Na+). In uncharged and -1 phosphates, the C-O-P-O(-C) torsion angles prefer the +/-sc regions. These phosphates substituted by cyclic C-sp3 noncyclic C-sp3, or C-ar carbons have characteristic torsion distributions. Conclusion from both analysis of crystal data and from theoretical calculations is that the internal phosphate geometry is sensitive to a counterion position as well as to values of the C-O-P-O(-C) torsion angles. Most metal cations interact directly with the charged oxygens in -1 phosphates while in many -2 phosphates, this interaction is mediated by water molecules. The distributions of Na+ around -1 and -2 phosphates are localized into two principal sites which Lie outside the O=P=O plane and interact with only one of the charged oxygens. In contrast, theoretically predicted positions are located symmetrically between the charged oxygens in the O=P=O plane. The cation positions around dimethyl phosphate and a model diPh compound mimicking the B-DNA backbone were virtually identical. The discrepancy between these theoretical positions and the sites derived from the crystal data was significantly reduced by incorporation of a single water molecule into the theoretical model. Therefore, both cations and polar particles, particularly water molecules, should be considered when properties of the phosphate group are described.