Abstract The crystal structure of Rb 3H(SeO 4) 2 in its “intermediate” phase has been determined at T=449 K using synchrotron radiation ( λ=1.00798 Å). The phase is monoclinic, space group C2/ m, with a=10.691(3) Å, b=6.167(2) Å, c=8.429(4) Å, β=115.00(1) V=503.6(4) Å 3, M r =543.32, D x =3.58 g cm −3, μ=81.3 cm −1, and Z=2. Refinement was carried out to a residual R w =0.049 for 1002 unique reflections with I>3 σ( I). Optical studies revealed the formation of a complex domain structure within this phase, and refinements were thus carried out assuming a twinned crystal model. Hydrogen bonds are arranged in a one-dimensional chain extending along b. Such a network is intermediate between that found in Rb 3H(SeO 4) 2 at room temperature, in which selenate groups are linked into well-defined dimers (zero-dimensional), and that found in Rb 3H(SeO 4) 2 at high temperature, in which the network is two-dimensional. The highly twinned structure suggests that macroscopic crystals will exhibit isotropic conductivity within the ab plane (similar to the high-temperature, trigonal phase) despite the anisotropic hydrogen bond arrangement within individual domains.