The electrical conductivity of LaP3O9 doped with I mol% of divalent metals (Ca, Sr, and Ba) substituting La has been investigated at p(H2O)=0.1 -5 kPa, p(O-2)=0.01 - 100 kPa and T= 573 -973 K. H/D isotope effects, proton transport number measurements, and independence of conductivity on p(O-2) showed that acceptor-doped LaP3O9 exhibits predominantly proton conduction. The conductivity was largely independent of p(H2O) at constant temperature, indicating that the acceptor was charge compensated by protons and exhibited protonic conduction even at low p(H2O). The conductivities depended considerably on the dopant species and increased in the order Ca < Ba < Sr. (c) 2005 Elsevier B.V. All rights reserved.