The structural behavior of the n = 1 Ruddlesden-Popper (R-P) La1.2Sr0.8MnO4+δ phases has been monitored in situ in flowing O2 on heating over the temp. range 65 \textless T/°C \textless 550 by means of neutron powder diffraction (instrument D20, ILL/Grenoble). Sequential Rietveld refinement showed that the I4/mmm nearly stoichiometric phase undergoes a first oxygen uptake at 300-400 °C with phase sepn. into an oxygen-rich orthorhombic (Bmab) La1.2Sr0.8MnO4.30(1) phase and a stoichiometric tetragonal (I4/mmm) La1.2Sr0.8MnO4.00(2) phase. At T ∼ 410 °C, the first oxidn. step is complete and only the Bmab oxidized phase is present. The orthorhombicity decreases progressively on further heating, up to 510 °C with minor variation in the oxygen content. Above ∼510 °C, the system undergoes a second abrupt oxidn. step involving a single phase process to reach after prolonged isothermal heating at 550 °C, δ ∼ 0.42(2). Such a high soly. for the excess-oxygen defects has never been reported so far for a K2NiF4-type structure. The La1.2Sr0.8MnO4+δ system is thus interesting for the fundamental studies of structural distortions induced by the intercalation of a large amt. of oxygen defects owing to the flexibility of the R-P structure and high stability of the Mn4+ oxidn. state in an oxygen octahedral environment. [on SciFinder(R)]