SiS2 and SiSe 2 are orthorhombic and isostructural, Ibam, with a = 9.583 (3), b = 5.614 (2), c = 5.547 (2),A,, V = 298.4 A 3, Z = 4, D x = 2.052 Mg m -3 at 293 K, a = 9.545(3), b = 5.564(2), c = 5.552 (2) A, V= 294.9 A 3, D x = 2.077 Mg m -3 at 138 K for SiS 2, and a = 9.669 (3), b = 5.998 (2), c = 5.851 (2) A, V = 339.3/k 3, Z = 4, D x = 3.640 Mg m -3 at 293K, a= 9.619 (3),b=5.989(2),c=5.851(2) .A, V = 337.1/~3, Dx = 3.665 Mg m -3 at 138 K for SiSe 2. The refinements-at 138 K (SiS 2) and 293 K (SiSe2) confirm the known structure types with some significant changes in cell dimensions and atomic coordinates. Final R factors for SiS2 (SiSe 2) are 0.041 (0.018) for 200 (253) observed reflexions. Within the chains of edge-sharing SiS~ (SiSe4) tetrahedra, the SiS (SiSe) bond lengths are 2.133 (I)A [2.275 (1)A], the S-Si-S (Se-Si-Se) bond angles in the planar fourmembered Si2S / (Si2Se2) rings are 98.8 (l)° [ 100.0 (1)°l.