The pressure effect on the superprotonic phase transition temperatures is studied for the series of [(NH4)(x)Rb1-x](3)H(SO4)(2) mixed crystals, x = 0, 0.2, 0.5, 0.8, and 1. It is shown that the R (3) over barm superprotonic phase does not occur in Rb3H(SO4)(2) at atmospheric pressure, but becomes stable at a pressure greater than 0.14 GPa. The scaling correlation is found between the A2/a-R (3) over barm transition lines in all P-T phase diagrams studied.