Two modifications of (TeO)(HAsO4) were obtained by reacting tellurium dioxide with arsenic acid under boiling conditions (modification I, acid concentration 80 wt-%) or under hydrothermal conditions (modification II, acid concentration 50 wt-%). The crystal structures of the two modifications were determined from single-crystal X-ray data [modification I: P21/c, Z = 4, a = 7.4076(10), b = 5.9596(7), c = 9.5523(11) Å, β = 102.589(8)°, 2893 structure factors, 68 parameters, R[F2 > 2σ(F2)] = 0.0247, wR2(F2 all) = 0.0530; modification II: P21/c, Z = 4, a = 6.2962(4), b = 4.7041(3), c = 13.9446(8) Å, β = 94.528(3)°, 2549 structure factors, 69 parameters, R[F2 > 2σ(F2)] = 0.0207, wR2(F2 all) = 0.0462)]. Dehydration of (TeO)(HAsO4)-II at temperatures above 260 °C results in the formation of polycrystalline (Te3O3)(AsO4)2. Single crystals of the anhydrous product were grown either by heating stoichiometric amounts of the binary oxides TeO2 and As2O5 in closed silica glass ampoules or with higher concentrated arsenic acid (80 wt-%) under hydrothermal conditions at 220 °C. The common features in the crystal structures of (Te3O3)(AsO4)2 [P, Z = 4, a = 6.5548(4), b = 7.6281(6), c = 15.0464(15) Å, α = 140.212(6), β = 102.418(9)°, γ = 77.346(5)°, 5718 structure factors, 146 parameters, R[F2 > 2σ(F2)] = 0.0351, wR2(F2 all) = 0. 1093] and in that of the two modifications of acidic (TeO)(HAsO4) are [TeO5] square-pyramids and [AsO4] tetrahedra. In anhydrous (Te3O3)(AsO4)2 and in (TeO)(HAsO4)-II, a layered arrangement of the building units is found, whereas in the (TeO)(HAsO4)-I structure strands are formed. Different hydrogen bonding interactions are present in the two modifications of (TeO)(HAsO4).