Calorimetric (DSC), NMR and electrical studies of Cs1.5Li1.5X(SO4)(2) (X = H, D) single crystals have been performed in the temperature range from 300 to 533 K. No phase transitions are observed upon heating and the conductivity follows an Arrhenius temperature dependence to the point of decomposition at similar to 470 K. Despite the high protonic conductivity of Cs1.5Li1.5X(SO4)(2) (sigma similar to 10(-3) S cm(-1)) at high temperature, these compounds cannot be classified as superprotonic because of the large value of the activation energy (E-a similar to 1 eV), The proton NMR studies confirmed that two crystallographic proton sites exist in Cs1.5Li1.5H(SO4)(2), presumably corresponding to two minima in the single, crystallographically distinct (and asymmetric) hydrogen bond. A measurable isotope effect in the conductivity data confirmed that protons/deuterons, as opposed to lithium ions, are the mobile species. The high activation energy for proton transport in CS1.5Li1.5X(SO4)(2) Most probably results from the asymmetry of the hydrogen bonds, and from electrostatic repulsion between the protons and the Li+ ions which likely hinders reorientation of XSO4- groups.