Mixed-valence arsenic / High pressure synthesis / Powder diffraction structure analysis / X-ray diffraction / Synchrotron radiation Abstract. We report on a new mixed valence arsenic(III,V)oxoacid H 6 As 3þ 7 As 5þ 7 O 31 which crystallizes in the hexagonal space group P6 3 (no. 173), with lattice parameters a ¼ 12.0525(1) A, c ¼ 4.7344(1) A, V ¼ 595.6(1) A 3 and Z ¼ 2. This phase is metastable and upon heating the RT-phase of As 2 O 5 is recovered. The structure exhibits the first As 5þ atom with a fivefold coordination by oxygen atoms in the shape of a distorted square pyramid. The positions of the hydrogen atoms were determined during the structure refinement from high resolution synchrotron powder diffraction data by difference Fourier analysis.