Single phase La_2xSr_xMnO_4±δ (0.7 x 1.0) solid solutions were prepared via sol-gel synthesis. For all the compositions the structure turns out to be tetragonal I4/mmm. Samples were then heated for 12 h at different temperatures in O₂ to investigate the structural response to oxygen uptake. On increasing the La/Sr ratio the maximum amount of oxygen inserted (δ) reaches for x = 0.7 an unprecedented value of δ ~ 0.47 in K₂NiF₄-structure type oxides, as evidenced from iodometric titration and thermogravimetric analysis. For the x 0.8 compositions the oxidation resulted in an intermediate orthorhombic phase.