Pleochroistic crystals (dark green to colorless) of a mixed-valence V(IV)-V(V) compound, K-6(VO)(4)(SO4)(8), suitable for X-ray determination have been obtained from the catalytically important K2S2O7-V2O5/SO2-O-2-SO3-N-2 molten salt-gas system, at similar to 400 degrees C. The compound crystallizes in the monoclinic space group P2(1) (No. 4) with 8.931(2) Angstrom, b = 18.303 (3) Angstrom, c = 9.971(2) Angstrom, beta 90.11(2)degrees, and Z = 2. It contains two rather similar V(IV)-V(V) pairs of VO6 octahedra distorted as usual having a short V-O bond of around 1.57 Angstrom, a long bond of around 2.40 Angstrom trans to this, and four equatorial bonds around 2.00 Angstrom. The bond lengths of the (VO6)-O-V octahedra are significantly shorter than those found for the (VO6)-O-IV octahedra. The eight different SO42- groups are all bridging bidentate between the V(IV) and V(V) atoms; a third oxygen is coordinated to a vanadium atom of a neighboring chain trans to the short V=O bond, and the fourth oxygen remains uncoordinated. The measured bond distances and angles show a considerable distortion of the SO4 tetrahedra. This is confirmed by the IR spectra of the compound, where large shift and splitting of the sulfate nu(3) bands UP to wave numbers of around 1300 cm(-1) is observed. The ESR spectra of the compound exhibit weak anisotropy with g(iso) 1.972 +/- 0.002 and Delta B-pp, = 65 +/- 2 G. The compound may cause the deactivation for industrial sulfuric acid catalysts observed around 400 degrees C in highly converted SO2-O-2-N-2 gas mixtures.