Electrical conduction in Ca-doped YPO4 with the tetragonal xenotime-type structure was investigated by using conductivity and thermoelectric power measurements under 0.4-8 kPa of p(H2O) and 0.01-100 kPa of p(H2O) at 500-930 degreesC. The conductivity of the 3 mol% Ca-doped YPO4 was 4 x 10(-6) to 2 x 10(-4) S cm(-1) in moisturized conditions at 500-930 degreesC. From humidity dependence and H/D isotope effect of the conductivity, it was found that YPO4 exhibited protonic conduction in moisturized conditions by substituting Ca for Y. Results of conductivity and thermoelectric power measurements versus p(H2O) and p(O-2) showed that protonic conduction in Ca-doped YPO4 was rather dominant, and that contribution of other ions or electronic conduction to the total conductivity was regarded as negligible. The magnitude of H/D isotope effect was from 1.3 to 1.6 and the activation energy for the D2O-treated sample was 0.03 eV higher than that for the H2O-treated sample, indicating semiclassical behavior in the free proton-hopping process in the material. (C) 2003 Elsevier B.V. All rights reserved.