Electrical conduction in rare earth orthophosphates, LnPO(4) (Ln = La, Pr, Nd and Sm), with the monazite structure (P2(1)/n) was investigated by using conductivity measurements at 500-925 degreesC. From the conductivities of undoped and 1 mol% Sr-doped LnPO(4) under wet (H2O and D2O) and dry atmospheres, it was found that LnPO(4) began to conduct protons under wet atmosphere by substituting Sr for Ln. The conductivity behavior of 1 mol% Sr-doped LnPO(4) versus p(H2O) and p(O-2) was discussed in terms of the defect equilibria. It was concluded that protonic conduction was dominant in the materials though electron holes contributed slightly to the total conductivity as temperature increased. All the 1 mol% Sr-doped LnPO(4) investigated in this study exhibited similar electrical conduction regardless of rare earth element used. (C) 2003 Elsevier B.V. All rights reserved.