Fully oxygenated perovskites with the same chemical composition crystallize in different polymorphs, o-LaCa2Fe3O9 with A-site triple-layer order and d-(LaCa2) Fe3O9 with A-site disorder. Both compounds contain unusually high valence Fe3.67+ and, when they are heated, release oxygen to reduce such an unusual valence state. Thermogravimetry analysis revealed that the ordered/disordered arrangements of the A-site cations in the perovskite structures strongly influence the stability of unusually high valence Fe ions at the B-site. The A-site-ordered o-LaCa2Fe3O9 releases its oxygen topotactically and selectively from the FeO6 octahedra between the Ca layers above 400 degrees C, and the released oxygen is not incorporated back on cooling in air, Ar, or O-2 atmospheres. On the other hand, oxygen in d-(LaCa2)Fe3O9 is released from and incorporated into the rigid octahedra reversibly when the compound is heated in air or O-2. More importantly, the oxygen-deficient d-(LaCa2)Fe3O8 obtained by heating d-(LaCa2)Fe3O9 in Ar incorporates extra oxygen to increase the valence state of Fe in an unusually high value even under ambient conditions. Once the A-site cation disorder structure framework is established, unusually high valence states, which usually require extreme conditions, can be stabilized by incorporating extra oxygen into the structure even under ambient conditions.