High temperature protonic conduction in 1 mol% Sr-doped (La1-xSmx)PO4 (x=0.3, 0.5 and 0.7) with the monazite structure (S. G.: P2(1)/n) was investigated. The conductivities of the materials under wet atmosphere were considerably higher than those under dry atmosphere. From the conductivity measurements under H2O- and D2O-containing atmospheres, it was found that the magnitude of H/D isotope effect was 1.2-1.4. These results indicated that protonic conduction was dominant in the materials under wet atmosphere. The p(H2O) dependencies of the conductivities were discussed in terms of the defect equilibria, and, as a result, higher Sm content in 1 mol% Sr-doped (La1-xSmx)PO4 seemed more preferable for proton dissolution. (C) 2004 Elsevier B.V. All rights reserved.