La2NiO4+δ (L2NO4) is a promising cathode material for low temperature solid oxide cells due to its good mixed electronic and ionic conductivity and high oxygen exchange activity with a low activation energy. L2NO4 has an oxygen over-stoichiometry in the range of 0 ≤ δ ≤ 0.18 due to its ability to accommodate excess oxygen as interstitials. The contribution from the interstitials is significant for the ionic conductivity of L2NO4 and the mobility of oxygen ions can be affected by the crystal structure. Below ca 450°C, several phases exist and/or coexist over this over-stoichiometry range. These phase transitions were studied between room temperature and 450 °C using Raman spectroscopy in a specific high temperature cell. Indeed, this temperature range corresponds to the one targeted for the use of this material in prospective fuel cells designed to work at quite low temperatures, and also corresponds to the interesting zone of the O-doping -  temperature phase diagram. A powder sample with δ ≈ 0.16, having a orthorhombic F4mmm structure was reduced in 10% H2/Ar atmosphere till the orthorhombic Bmab structure (0 ? δ ? 0.01) was obtained. Another phase with its own characteristic spectrum, along with phase mixtures were also observed. The experiment was reproduced by annealing the reduced powder in air and observing the phase transitions over the range of the over-stoichiometry attainable with the temperature and O partial pressure conditions used. Thin films of 40 to 200 nm, deposited by pulsed injection-Metal Organic Chemical Vapor Deposition (MOCVD), were also analyzed using the same protocol. Results are tentatively described in terms of microtexture (film porosity) and material grain size. This study shows the feasibility of using in situ Raman spectroscopy to study the phase diagram of bulk and thin film L2NO4.