The crystal structure of Rb3H(SeO4)(2) in its "intermediate" phase has been determined at T=449 K using synchrotron radiation (lambda = 1.00798 Angstrom). The phase is monoclinic, space group C2/m, with a 10.691(3) Angstrom, b = 6.167(2) Angstrom, c = 8.429(4) Angstrom, beta = 115.00(1) V = 503.6(4) Angstrom(3), M-r = 543.32, D-x = 3.58 g cm(-3), mu = 81.3 cm (-1) and Z = 2. Refinement was carried out to a residual R-w = 0.049 for 1002 unique reflections with I>3sigma(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 Rb3H(SeO4)2 at room temperature, in which selenate groups are linked into well-defined dimers (zero-dimensional), and that found in Rb3H(ScO4)(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.