A dense ceramic La2NiO4+delta membrane was annealed under an oxygen potential difference Delta mu(O2) between flowing O-2 and Ar at 1000 degrees C for 800 h. The membrane was afterwards analyzed by means of SEM, EPMA, XPS and XRD. SEM and XPS analyses showed that NiO was segregated and randomly spread as new decomposition phase on the high pO(2) side. Pores had formed on the low pO(2) side without detectable phase change. The critical Delta mu(O2) to initialize the decomposition of the LNO membrane was derived and expressed by kinetic and thermodynamic parameters. Based on the calculated critical Delta mu(O2) and the observed experimental results, the ratio of effective diffusivities DLa/DNi in the ceramic material at 1000 degrees C was estimated to be slightly lower than 0.5. The oxygen flux remained mainly constant after an initial decrease during the annealing. Results from this work are in good agreement with our findings on cation diffusivity in La2NiO4+delta material obtained by solid state reaction, chemical tracer diffusion and inter-diffusion. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.055208jes]