The effect of the thermal treatment on the superprotonic phase transition (T-sp similar or equal to 348 K) and protonic transport of the Cs5H3(SeO4)(4) . xH(2)O single crystals was studied. It was found that annealing at T > T-sp results in the suppression of the phase transition to the low-conductivity state. This irreversibility of the superprotonic phase transition is due to changes in the water content. A model of the water loss process is proposed.