The crystal structures among M1-M2-(H)-arsenites (M1 = Li + , Na + , K + , Rb + , Cs + , Ca 2+ , Sr 2+ , Ba 2+ , Cd 2+ , Pb 2+ ; M2 = Mg 2+ , Mn 2+,3+ , Fe 2+,3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) are less investigated. Up to now, only the structure of Pb 3 Mn(AsO 3 ) 2 (AsO 2 OH) was described. The crystal structure of hydrothermally synthesized Na 4 Cd 7 (AsO 3 ) 6 was solved from the single-crystal X-ray diffraction data. Its trigonal crystal structure [space group R3, a = 9.5229(13), c = 19.258(4) Å, γ = 120°, V = 1512.5(5) Å 3 , Z = 3] represents a new structure type. The As atoms are arranged in monomeric (AsO 3 ) 3units. The surroundings *