The diarsenides of the cobalt and nickel families have been synthesized for magnetic susceptibility studies. The diffraction pattern of the low-temperature modificatioil of nickel diarsenide differs from that of the lnineral pararailln~elsbergite, although the space group is apparently idelltical and the unit cell dimensions comparable (orthorhombic Dig-Pbca; a = 5.770 f 0.007, b = 5.838 f 0.006, c = 11.419 f 0.008 A). These binary diarsenides exhibit temperature dependent diamagnetic or weakly paramagnetic behavior. Temperature dependence of electrical resistivity indicates that PdAs? is metallic while PtAs? is a semiconductor with a n energy gap of -0.17 eV.Substitutional solid solution limits have been determined for various Group VIII transition metal atoms in the pyrite phases PdAs? and PtAs?, and in the skutterudites RhAss and IrAs3. There is no substitution by Ru or 0 s in PdAse, by Ni or P t in PdAs?, by Ru or Ag in RhAs3, or by 0 s in IrAs3. Very limited substitutions were observed for Rh and Ag in PdAs?, Pd in I'tAs?, Pd in Rh.4~3, and I' t in IrAss. The limit of Co substitution in PdAs? is 8.2 f 0.3atoln 7 0 , and of Ni in I'dAs?, 56.7 f 0.3 atom %.111 the Co,Pdl-,As? system, a temperature-dependent paramagnetic colltribution corresponding to approximately one uncompensated spin per cobalt ion has been attributed to the t5?, loiv-spin configuration of the Co ion in the pyrite structure. In the Ni,Pdl-,As2 system, the temperature-independent paramagnetic contribution by the nickel ions is not colnpletely ~inderstood.