Single crystals of (K-1 (-) (x)(NH4)(x))(m)H-n(SO4)((m + n)/2)center dot yH(2)O solid solutions are grown in the K3H(SO4)(2)-(NH4)(3)H(SO4)(2)-H2O system which end members are known to undergo superprotonic phase transitions with essentially different kinetics. Replacements in the cation sublattice and their influence on structural transitions as well as dielectric and optical properties of these crystals are of obvious interest both for fundamental and applied physics as new functional materials. Solutions of K3H(SO4)(2) and (NH4)(3)H(SO4)(2) with different ratio (K:NH4) crystallize in different structure types. The ratio 1:1 leads to the single crystals, which have the general formula ((K1 - x(NH4)(x))(9)H-7(SO4)(8)center dot H2O. The grown crystals are characterized and the crystal structure of the (K1 - x(NH4)(x))(m)H-n(SO4)((m + n)/2)center dot H2O (X = 0.04) crystal is solved. Data comparison for (K1 - x(NH4)(x))(m)H-n(SO4)((m + n)/2)center dot H2O and K9H7(SO4)(8)center dot H2O crystals allows to establish structural conditionalities of differences in conductivity. One can assume that potassium ions are involved in the conductivity in addition to protons. (C) 2014 Elsevier B.V. All rights reserved.