The crystal structures of chalcostibite CuSbS2 (orthorhombic, space group Pnma, a = 6.018(1), b = 3.7958(6), and c = 14.495(7) Å, V = 331.1(1) Å3, Z = 4, R1 = 0.040, wR2 = 0.155 for 533 reflections) and emplectite CuBiS2 (orthorhombic, space group Pnma, a = 6.134(1), b = 3.9111(8), and c = 14.548(8) Å, V = 348.8(2) Å3, Z = 4, R1 = 0.037, wR2 = 0.112 for 492 reflections) were redetermined using a four-circle diffractometer and graphite-monochromatized MoKα radiation. These two crystal structures are composed of MS5 square pyramids (M = Sb and Bi) and nearly regular CuS4 tetrahedra. The five M-S bond distances in the SbS5 square pyramid in chalcostibite are always shorter than corresponding distances in the BiS5 square pyramid in emplectite because the Sb atom is smaller than the Bi atom. The a cell parameter increases appreciably from chalcostibite to emplectite not only because of increasing M-S bond distances in the MS5 square pyramid, but also because of increasing Cu-S2-Cu bond angles along a. The increase in the b cell parameter is caused mainly by increasing M-S bond distances along b. In contrast, the slight increase of the c cell parameter is largely brought about by decreasing Cu-S2-Cu bond angles ascribed to weakened stereochemical activity of Bi 6s2 lone-pair electrons. The anisotropic change of unit-cell parameters from chalcostibite to emplectite is strongly associated with the positions of the lone-pair electrons in the unit cell.