The chemical shift (CS) tensors of two four-membered heterocyclic diphosphetes, trans- and cis-1,2-dihydro-1-methyl-2-phenyl-3,3-bis(tert-butyl)-1,2-diphosphete, have been investigated by solid-state P-31 NMR spectroscopy. The dipolar chemical shift method has been employed to determine the magnitudes of the principal components of the P-31 CS tensors in these compounds, as well as their orientations relative to the (31)p-(31)p dipolar vector. The spans of the phenyl- and methyl-substituted phosphorus CS tensors of the trans isomer are 248 and 280 ppm, respectively, significantly larger than the corresponding values for the cis isomer, 139 and 173 ppm. The orientations of the CS tensors in the molecular framework are proposed on the basis of ab initio calculations using the gauge-independent atomic orbitals method. The combined experimental-theoretical results suggest that, for both the trans and cis isomers, the least shielded principal components, delta(11), are oriented in the plane defined by the four membered ring. For the trans isomer, the most shielded principal components, delta(33), are approximately perpendicular to this plane; however, for the cis isomer, it is the delta(22) components that are perpendicular to the plane of the four-membered ring. The spectra of magic-angle spinning samples of the trans isomer exhibit asymmetric, spinning-frequency dependent line shapes, while those of the cis isomer are invariant to spinning frequency. These observations are consistent with the CS tensors derived from the P-31 NMR spectra of stationary samples.