A single-crystal X-ray diffraction study at 110 K and ab initio calculations up to the MP2/6-31G* level of theory showed that the ground state of P(NMe(2))(3) has C-s symmetry, with two different coordination geometries at the nitrogen atoms (pyramidal and planar). The calculated and experimentally determined geometries are in good agreement. Steric strain could be ruled out as the predominant factor causing the deviation from the expected C-3 symmetry, because ab initio calculations on the model system P(NH2)(3) (up to MP2/6-311G**) also indicate C-s symmetry for the ground-state structure, whereas calculations on N(NH2)(3) have predicted this molecule to have C-3 symmetry. The structure of the phosphorane H2C=P(NMe(2))(3) has been elucidated by X-ray diffraction in the solid state (110 K) and by electron diffraction in the gas phase augmented by restraints derived from ab initio calculations up to the MP2/6-31G* level of theory. Solid-state and gas-phase structures are in good agreement showing again the C-s arrangement to be the ground-state structure of the molecule. Ab initio calculations (MP2/6-311G**) indicate that H2C=P(NH2)(3) also has C-s symmetry. The reasons for the preference for C-s rather than C-3 symmetry adopted by the P(NR(2))(3) units are discussed in terms of steric repulsion, lone pair, and other electronic interactions. Literature reports of structures of derivatives containing P(NMe(2))(3) units are discussed in the light of the new results. Selected structural results include : P(NMe(2))(3) (XRD, average values); P-N(1/2) 1.687, P-N(3) 1.731 Angstrom, N(1)-P-N(2) 110.8, N(3)-P-N(1/2) 97.7 degrees, Sigma angle at N(1/2) 355.9, at N(3) 337.6 degrees; H2C=P(NMe(2))(3) (GED/XRD average without esd); P-C(1) 1.620(5)/1.655(6), P-N(1/2) 1.684(3)/1.668, PN(3) 1.718(6)/1.698(4) Angstrom, N(1)-P-N(2) 115.2(13)/ 114.7(2), N(1/2)-P-N(3) 97.0(5)/99.6, C(1)-P-N(1/2) 110.0(5)/110.0, C(1)-P-N(3), 127.1(8)/122.4(4)degrees, Sigma angle at N(1/2) 359.5(11)/353.4, at N(3) 332.9(13)/337.3(6)degrees.