Abstract Two organically templated metal arsenates, [NH 3(CH 2) 3NH 3] 0.5[ M(OH)AsO 4] ( M=Ga, Fe), have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction and 71Ga NMR spectroscopy. The two compounds are isostructural and crystallize in the orthorhombic space group Fdd2 (No. 43) with a=9.5584(7) Å, b=41.593(3) Å, c=5.6438(4) Å, V=2243.8(3) Å 3, and Z=16 with R 1=0.0302 for the Ga compound, and a=9.7301(2) Å, b=41.5035(2) Å, c=5.7411(1) Å, V=2318.45(8) Å 3, and Z=16 with R 1=0.0228 for the Fe compound. The structure consists of two-dimensional anionic sheets of metal arsenate in the ac plane with charge-compensating propanediammonium cations between the sheets. The sheets are constructed from infinite chains of cis-corner-sharing MO 6 octahedra, which are joined by arsenate ligands. This work is one of the few studies concerning organically templated metal arsenates.