A single crystal X-ray diffraction investigation of the superionic conductor Cs0.9Rb0.1HSO4 has been performed at room temperature with the aim of studying the influence of the introduction of a heteroalkali metal ion on the crystal structure with respect to that of the end members, and its influence on the temperature of the phase transition to the superionic phase. M(r) = 225.23, lambda(MoKalpha) = 0.70930 Angstrom, monoclinic, P2(1), a = 7.752(2) Angstrom, b = 8.117(2) Angstrom, c = 14.577(4) Angstrom, beta = 99.09(2)degrees, V = 905.7(4) Angstrom(3), Z = 8, D-x = 3.29 g cm(-3), mu = 90.2 cm(-1), F(000) = 818, R(F) = 0.034, R(F)(w) = 0.034 for 2009 unique reflections. Inclusion of a small amount of rubidium ions is sufficient to perturb the metastable room temperature phase III structure of the end member CsHSO4, and the structure can be considered instead as similar to that of phase II, usually stable only above 330 K. Cs0.9Rb0.1HSO4 exhibits a strictly one-dimensional character, with sulfate groups being linked into zigzag chains through hydrogen bonds as short as 2.48(2) Angstrom.