Over the past few years, the use of nickelates as the cathode material in solid oxide fuel cells (SOFCs), has attracted a growing attention due largely to their superior properties to some commercialized perovskite materials. However, the nickelate does go through phase transition at operation temperatures. To improve the stability of Pr2NiO4, Nd was used to substitute Pr to form (Pr_1-xNd_x)₂NiO₄ (PNNO) which exhibits an improved phase stability. The phase degradation was found to accelerate when the PNO cathode is under an operation. It is known that the apparent oxygen partial pressure at the cathode/electrolyte interface is lower than that in cathode gas stream. In order to investigate the relationship between the accelerated phase transition and oxygen partial pressure, PNNO oxides are annealed under different P_O2 conditions. Multiple post-analyses are carried out on these samples to obtain a quantitative relationship between the phase composition and P_O2 during the annealing process.