Crystals of Ph]2 are triclinic, a= 8"06, b=9"34, c= 13.13 A, ~=93"6, t= 104.2, y=99.2 °, space group P], 2 molecules in the unit cell. The atomic positions have been determined by least-squares refinement from X-ray diffractometer intensity data, the final R being 0.095 for 3101 reflexions. The two molecules in the unit cell lie on centres a.J of symmetry at (0,0,0) and (2, z-,½) and in consequence the phosphazane ring is planar. These molecules, although not mutually related by symmetry, differ in shape only by the position of the terminal carbon atom of the ethyl group. Mean bond lengths and angles are: P-N 1.688, P-S 1.928, P-C 1-807, N-C 1.47 ~, N-P-N 83.25 °, and P-N-P 96.75 °. The geometry of the molecule is discussed in terms of steric factors.