Complex oxides occurring in the systems CoO/AS2O5and NiO/As2O5have been studied using Debye–Sherrer diagrams. The compounds were prepared either by high-temperature reaction of the oxides and subsequent pyrolysis of the products or by aqueous precipitation of arsenates followed by thermal dehydration.The existence of compounds having metal oxide to arsenic pentoxide ratios of 1:1, 2:1, and 3:1 has been clearly demonstrated and the thermal stabilities of eachinvacuoand in air have been studied. Corresponding compounds in the cobalt and nickel series are isostructural and unit-cell dimensions have been determined. The compounds with a 2:1 ratio could not be isolated readily although their existence is indisputable and tentative figures for the unit-cell dimensions have been obtained.Another compound, probably with a metal oxide to arsenic pentoxide ratio of 6:1, has been observed during the pyrolysis of the 3:1 compounds. Apparently the cobalt and nickel compounds are isostructural and represent the only phase existing between the 3:1 compounds and the metal oxides.