Summary: Data are presented from cobaltite (CoAsS), gersdorffite (NiAsS), and ullmannite (NiSbS) for cell size, compositional zoning, mineralogical association, X-ray diffraction powder intensities, and optical anisotropy. The presence of powder X-ray diffraction reflections 001 and 011 are related to the crystal structure type. The amount of distortion from cubic symmetry is related to the 001 reflection intensity and also to the optical anisotropism strength. These properties are related approximately to the thermal stability of the distorted crystal structure. This thermal stability increases with the substitution of cobalt for nickel, which decreases the cell size. An order-disorder change occurs before the distortion release in non-cubic gersdorffite (P1) with a large cell size, whereas only a distortion release is observed in cobaltite (Pca21) and non-cubic gersdorffite (P1) with a small cell size. Glaucodot (Co,Fe)AsS converted to cobaltite at 620°C. No evidence was found to relate optical anisotropism of pyrite to deviations from cubic symmetry.