P-31 chemical shift anisotropy (CSA) tensors have been calculated for a set of selected DNA and RNA backbone conformations using density functional theory. The set includes canonical A-RNA, A-DNA, B-I-DNA, B-II-DNA, Z(I)-DNA, and Z(II)-DNA as well as four A-RNA-type, seven non-A-RNA-type, and three non-canonical DNA conformations. Hexahydrated dimethyl phosphate has been employed as a model. The P-31 chemical shift tensors obtained are discussed in terms of similarities in the behavior observed for gauche-gauche (gg) and gauche-trans (gt) conformations around the P-O bonds. We show that torsion angles alpha and zeta are major determinants of the isotropic chemical shift delta(iso) and of the delta(CSA)(11) component of the traceless chemical shift tensor, which is revealed in separate ranges of both delta(iso) and delta(CSA)(11) for gg- and gt-conformers, respectively. A clear distinction between the two conformation types has not been found for the delta(CSA)(22) and delta(CSA)(33) components, which is attributed to their different directional properties. The P-31 CSA tensors exhibit considerable variations resulting in large spans of similar to 16 ppm for delta(CSA)(11) and similar to 22 ppm for delta(CSA)(22) and delta(CSA)(33). We examine the consequences of the CSA variations for predicting the chemical shift changes upon partial alignment delta(CSA) and for the values of CSA order parameters extracted from the analysis of P-31 NMR relaxation data. The theoretical P-31 CSA tensors as well as the experimental P-31 CSA tensor of barium diethyl phosphate (BDEP) are used to calculate delta(CSA) for two eclipsed orientations of the CSA and molecular alignment tensors. Percentage differences between the CSA order parameters obtained using the theoretical P-31 CSA tensors and the experimental P-31 CSA tensor of BDEP, respectively, are also determined.