The thermal evolution of the phase composition of CeP2O7 and Ce(PO3)(4) with 10 mol% Y and Gd doping, respectively, was examined by in-situ powder X-ray diffraction and thermogravimetry with in-line mass spectroscopy. The phase composition depends critically on the P to metal ratio, the annealing temperature, humidity and time. CeP2O7 and Ce(PO3)(4) were completely decomposed to CePO4 following a 1100 h long conductivity test at 155 degrees C. The conductivity of 10 mol% Gd doped Ce(PO3)(4) (synthesized with P:(Ce + Gd) = 5.0) reaches a value of 6.4 . 10(-2) S . cm(-1) at 150 degrees C under wet conditions (pH(2)O = 0.2 atm). The conductivity of 10 mol% Y doped CeP2O7 (synthesized with P:(Ce + Y) = 3.1) was 1.9 . 10(-2) S . cm(-1) under the same conditions. Long term conductivity measurements are reported here for the first time and the effect of repeated hydration-dehydration cycles on the conductivity is examined. Exsolution of PmOn and increase of the highly hygroscopic amorphous secondary phase significantly affects the conducting properties. KH2PO4 was observed to re-crystallize and form amorphous potassium phosphate at temperatures above 100 degrees C in the 10 mol% Y doped CeP2O7:KH2PO4 composite (synthesized with P:(Ce + Y) = 3.1) resulting in a conductivity value of 2.6 . 10(-2) S . cm(-1) at 150 degrees C and pH(2)O = 0.2 atm. (C) 2013 The Electrochemical Society. All rights reserved.