The diarsenides of the cobalt and nickel families have been synthesized for magnetic susceptibility studies. The diffraction pattern of the low-temperature modification of nickel diarsenide differs from that of the mineral pararammelsbergite, although the space group is apparently identical and the unit cell dimensions comparable (orthorhombica = 5.770 ± 0.007,b = 5.838 ± 0.006,c = 11.419 ± 0.008 Å). These binary diarsenides exhibit temperature dependent diamagnetic or weakly paramagnetic behavior. Temperature dependence of electrical resistivity indicates that PdAs2is metallic while PtAs2is a semiconductor with an energy gap of ~0.17 eV.Substitutional solid solution limits have been determined for various Group VIII transition metal atoms in the pyrite phases PdAs2and PtAs2, and in the skutterudites RhAs3and IrAs3. There is no substitution by Ru or Os in PdAs2, by Ni or Pt in PdAsa, by Ru or Ag in RhAs3, or by Os in IrAs3. Very limited substitutions were observed for Rh and Ag in PdAs2, Pd in PtAs2, Pd in RhAs3, and Pt in IrAs3. The limit of Co substitution in PdAs2is 8.2 ± 0.3 atom %, and of Ni in PdAs2, 56.7 ± 0.3 atom%.In the CoxPd1−xAs2system, a temperature-dependent paramagnetic contribution corresponding to approximately one uncompensated spin per cobalt ion has been attributed to thet52glow-spin configuration of the Co ion in the pyrite structure. In the NixPd1−xAs2system, the temperature-independent paramagnetic contribution by the nickel ions is not completely understood.