The knowledge of the thermal evolution of the crystal structure of a cathode material across the usual working conditions in solid oxide fuel cells is essential to understand not only its transport properties but also its chemical and mechanical stability in the working environment. In this regard, high resolution neutron powder diffraction (NPD) measurements have been performed in air from 25 to 700 °C on O₂-treated (350 °C, 200 bar) La₂NiO_4+δ. A structural transition from the orthorhombic Fmmm to the tetragonal F4/mmm space group takes place at about 150 °C. The reversibility of this transition has been determined to be strongly dependent on the sample oxygen content. The structural data have been correlated with the transport properties of this layered perovskite. The electrical conductivity of O₂-treated La₂NiO_4+δ exhibits a dirty-metal (high T)-to-semiconducting (low T) transition as a function of temperature, displaying a maximum value of 82 S cm⁻¹ at around 400 °C. The largest conductivity corresponds, microscopically, to the shortest axial Ni–O2 distance (2.19(1) Å), revealing a major anisotropic component for the electronic transport. The interstitial oxygens occupy the 16j and 16e positions in the low and high temperature phases, respectively. The refined oxygen occupancy from NPD data is in quite good agreement with the thermogravimetric data. Good thermal stability of the oxygen content has been observed in the studied temperature range, as required for practical applications.