The homogeneous substitution of cations (K+) and anions (H2PO 4 − ) for CsHSO4 is performed. The dependences of protonic conductivity and structure of (1 − x)CsHSO4-xKH2PO4 (x = 0.05–0.9) compounds on the composition are studied. It is found that the introduction of KH2PO4 leads to the formation of a new highly conductive phase. At small amounts of introduced KH2PO4 (x = 0.05), a mixed salt forms; its low-temperature conductivity is by more than two orders of magnitude higher than that of the source salts. The thermal behavior of mixed salt (1 − x)CsHSO4-xKH2PO4 of various compositions and the peculiarities of crystal structure are studied. The structural parameters of the salt at x = 0.05–0.5 are close to those of Cs3(HSO4)2(H2PO4). At higher x, another phase forms, whose structure has yet to be determined. The thermal stability of the salt decreases with increasing fraction of KH2PO4 introduced. The conductivity of the composites based on the mixed salt and silicon dioxide (1 − y){xKH2PO4-(1 − x)CsHSO4}-ySiO2 (x = 0.05, 0.1; y = 0.1–0.7) is studied. It is shown that, in the low-temperature range, the conductivity of composite systems increases within an order of magnitude, passes through a maximum, and, then, decreases at y > 0.5 due to the percolation effect.