(x/2)Sr2P2O7-(1-x)LaPO4 (x=0-0.60) was prepared with a solid state reaction between LaPO4 and SrHPO4, and its electrical conduction was investigated by using conductivity measurements under 0.4-5 kPa of p(H2O) and 0.01-100 kPa of p(O-2) at 500-930 degreesC. From humidity dependence and H/D isotope effect of the conductivity, it was found that (x/2)Sr2P2O7-(1-x)LaPO4 with x>0 conducted protons dominantly under moisturized conditions, whereas LaPO4 (x=0) did not exhibit protonic conduction. The conductivity of (x/2)Sr2P2O7-(1-x)LaPO4 showed a maximum when x=0.02-0.03, which was nearly equal to the solubility limit of Sr into LaPO4. These results suggested that substitution of Sr for La was essential to induce protonic conduction in Sr2P2O7-LaPO4 system and that existence of Sr2P2O7 as a secondary phase deteriorated the conductivity. Finally, p(H2O) and p(O-2) dependencies of the conductivity were discussed in terms of defect equilibrium. (C) 2004 Elsevier B.V. All rights reserved.