As part of a waste immobilisation study, single crystals of two cadmium arsenate phases have been prepared by hydrothermal methods. CdNH4(HAsO4) OH is orthorhombic with space group Pnma, a = 17.682( 4), b = 6.0098(12), c = 5.1917(10) Angstrom. Cd5H2(AsO4)(4).4H(2)O is monoclinic with space group C2/c, a = 18.382(2), b = 9.7192(13), c = 9.9791(13) Angstrom, beta = 96.369(3) degrees. Both structures were determined using single crystal X-ray diffraction and are isostructural with the analagous phosphates. CdNH4(HAsO4)OH easily converts to Cd5H2(AsO4)(4).4H(2)O on reaction with water and even in moist air. The latter phase is relatively stable by comparison and can be shown to continue to crystallise from amorphous material without substantial leaching of cadmium or arsenic.