In developing a new compositae air electrode for Solid Oxide Cells (SOCs) it is essential to fully understand the phase chemistry of all components. Ruddlesden-Popper type electrodes such as Pr2NiO4+delta have previously been proposed as attractive alternatives to conventional La0.6Sr0.4Fe0.8Co0.2O3-delta/Ce1-xGdxO2-delta compositae air electrodes for both fuel cell and electrolyser modes of operation. However, Pr2NiO4+delta have been shown to have limited stability, reacting with a Ce1-xGdxO2-delta interlayer to form a Ce1-x-yGdxPryO2-delta (CGPO) phase of unknown stoichiometry. Additionally, Pr2NiO4+delta are known to decompose to Pr4Ni3O10 +/- delta under certain conditions. In this work detailed understanding of the chemical reaction between Pr2NiO4+delta and Ce0.9Gd0.1O2-delta (CG010) under normal solid oxide cell fabrication and operating temperatures was obtained, identifying the composition of the resulting CGPO phase reaction products. It is shown that, in addition to the unreacted CGO10 present after sintering the compositae at 1100 degrees C for up to 12 h, a series of CGPO chemical compositions were formed with various Ce, Gd and Pr ratios depending on the relative distance of the doped ceria phases from the Pr2NiO4+delta phases. The extent of the chemical reaction was found to depend on the sintering time and the contact area of the two phases. Further thermal treatment of the resulting products under SOC air electrode operating temperature (800 degrees C) resulted in the initiation of Pr2NiO4+delta decomposition, forming Pr4Ni3O10 +/- delta and Pr6O11 with no detectable change in the composition of previously formed Pr-substituted ceria phases. It is apparent that the Pr2NiO4+delta/CGO10 compositae is unsuitable as an air electrode, but there is evidence that the decomposition products, Pr4Ni3O10 +/- delta and Ce1-x-yGdxPryO2-delta are stable and suitable candidates for SOC electrodes. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.