Electrical conduction in lanthanum orthoborates LaBO3 with the orthorhombic aragonite-type structure (S. G.: Pnam) 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 LaBO3 exhibited dominantly protonic conduction under wet conditions by substituting Sr for La. Results of conductivity measurements under various p(H2O) and p(O-2) suggested that the contribution of electron hole conduction became significant as temperature increased, P(O-2) increased and p(H2O) decreased, although protonic conduction was estimated as predominant under the conditions used. The conductivities of the 1 mol% Sr-doped LaBO3 were 1 x 10(-5) -2 x 10(-4) S cm(-1) under moisturized conditions at 500-930 degreesC. The conductivity behaviors as well as the conductivity values of the Sr-doped LaBO3 with the aragonite-type structure were quite similar to those of the acceptor-doped rare earth orthophosphates, indicating a similar proton conduction mechanism in these oxoacid salts. (C) 2004 Elsevier B.V. All rights reserved.