Two fluorinated metal arsenates, (C4H12N2)(1.5)[M3F5(HAsO4)(2)(AsO4)] (M = Fe, Ga), have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, magnetic susceptibility, Mossbauer spectroscopy, and Ga-71 NMR spectroscopy. The two compounds are isostructural and crystallize in the monoclinic space group P2(1)/c (No. 14) with a = 8.394(l) Angstrom, b = 21.992(3) Angstrom, c = 10.847(l) Angstrom, beta = 96.188(2)degrees, and Z = 4 for the Fe compound, and a = 8.398(l) Angstrom, b = 21.730(3) Angstrom, c = 10.679(l) Angstrom, beta = 95.318(2)degrees, and Z = 4 for the Ga compound. The structure consists of infinite chains of corner-sharing MX6 (X = O, F) octahedra and dimers of edge-sharing MO3F3 octahedra, which are linked into two-dimensional sheets through arsenate tetrahedra with diprotonated piperazinium cations between the sheets. Magnetic susceptibility and Mossbauer spectroscopy confirm the presence of Fe(III). The Ga-71 MAS NMR spectrum clearly shows a line shape consisting of three components, corresponding to three crystallographically distinct Ga sites.