The study by X-ray diffraction, calorimetry, vibrational and impedance spectroscopy of CsH(SO4)(0.76)(SeO4)(0.24) new solid solution is presented. Crystals of this composition undergo two phase transitions at T = 333 and 408 K. The last one at 408 K is a superionic-protonic transition (SPT) related to a rapid [HS(Se)O-4(-)] reorientation and fast H+ diffusion. A sudden jump in the conductivity plot confirms the presence of this transition. Above 408 K, this high temperature phase is characterized by high electrical conductivity (7 x 10(-3) Omega(-1) cm(-1)) and low activation energy (E-a < 0.3 eV).