dedicated to professor g. scheler on the occasion of his 70th birthday An intergrown crystal of two phases of bis(dineopentoxyphosphorothioyl) diselenide 1 was investigated by goniometer 31 P NMR. From the angular dependence of the chemical shift, the tensors of a triclinic and a monoclinic phase were determined. The principal values ฯ 11 , ฯ 22 , and ฯ 33 of the absolute nuclear magnetic shielding tensors for the triclinic phase are 134.1, 227.2, and 375.5 ppm and for the monoclinic phase are 132.4, 227.8, and 374.2 ppm, respectively. In both cases, the principal axis 3 of the 31 P tensor is directed nearly along the P S bond and the principal axis 2 is nearly perpendicular to the S P Se plane. Calculations of the 31 P and 77 Se nuclear magnetic shielding tensors were performed for molecules of both phases of 1 and for model compounds by the sum-over-states density functional perturbation theory IGLO method. The rms distances between calculated and experimental 31 P NMR icosahedral tensor values ฯ j (j = 1 6) amount to 17-21 ppm. The calculated and experimental orientations of the 31 P principal axes show a maximum difference of 5 โข and rms distances of 3.2 and 3 3 โข . For the principal value ฯ 33 of the selenium shielding tensor the agreement between calculated and experimental values is satisfactory, but the calculated values ฯ 11 and ฯ 22 are distinctly too small. Calculations for a model compound in which the methyl groups of the neopentoxy residue are substituted by protons lead practically to the same results.