Solid solutions of (CsHSO4)(1-x)(CsH2PO4)(x) (x=0.25-0.75) were synthesized by mechanical milling method over a wide range of compositions. Superprotonic cubic phase was confirmed for all these samples between 293 and 420 K depending on its composition. These superprotonic phases have primitive cubic structure similar to that of CsH2PO4. The kinetic stability of the supercooled cubic phase depends both on the composition x and the humidity of surrounding atmosphere. The most stable composition of the cubic phase was found around x=0.67 and could be maintained for several days even under ambient atmosphere. The ionic conductivities of these superprotonic phases reached 10(-2)-10(-3) S.cm(-1) at 450 K. With increasing x the ionic conductivity at the superprotonic phase decreased continuously associated with the increase of the activation energy. These findings suggest that the average number of the hydrogen bonds between XO4 (X = S, P) units plays an important role on the stability of the cubic phase and also on the conductivity. (c) 2008 Elsevier B.V. All rights reserved.