The vapour pressure of P4O10 produced by decomposition of LaP3O9 and LaP5O14 (LaP3O9 -> LaPO4 + 1/2 P4O10; LaP5O14 -> LaP3O9 + 1/2 P4O10) was measured by the transpiration method over the temperature ranges (1123 to 1323) K and (923 to 1173) K, respectively. The standard thermodynamic quantities of LaP3O9 and LaP5O14 were derived by the use of second-law method and Neumann-Kopp rule. The derived thermodynamic quantities are consistent with the phase equilibria in the (La2O3 + P2O5) system reported previously. There is a significant discrepancy between the present results and the literature data for LaP3O9 at T = 298.15 K, possibly due to the lack of accurate thermodynamic values of LaPO4 in the past. The thermal and chemical stabilities of LaP3O9 and LaP5O14 were analysed toward their proposed use as proton-conducting solid electrolytes in fuel cells. LaP3O9 is considered more promising from the viewpoint of proton conductivity and thermal stability. In the presence of water vapour or hydrogen gas, the decomposition of LaP3O9 and LaP5O14 would be accelerated significantly. (C) 2013 Elsevier Ltd. All rights reserved.