Mechanical mixtures of zirconia xerogel with variable content of crystalline Y2O3 up to 25 mol%, were hydrothermally treated by microwave route at 110 °C for 2 hours in the presence of 0.2 M solution of (KOH+K2CO3) mineralizer. The resulting amorphous hydrated zirconia-yttria solid solutions with a maximum solubility of Y2O3 content between 20 ~ 25 mol%, showed a remarkable reduction of the surface area at the increasing Y2O3 content of the starting mixture. The as-synthesized products and the corresponding calcined powders at 400 °C were uniaxially pressed into pellets (10 x 7 x 2 ~ 4 mm, in width) at 150 MPa. Conductivities were measured at 25 °C by AC impedance method with a frequency range from 10 Hz to 1 MHz with the pellets equilibrated either under silica gel or under increasing relative humidity (RH) up to ~90 %. The effects of composition, surface area, calcination temperature and relative RH on the proton conductivity of the amorphous solid solutions are discussed.