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Tuning the properties of transition-metal oxides by controlling oxygen content has value in many applications. $T\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ oxides, which can vary their crystal structure as oxygen content changes, are promising materials for reversible manipulation, since they do not create defects in random fashion. Real-time x-ray scattering is used to monitor the topotactic phase transition and its reversibility in SrFe${}_{0.8}$Co${}_{0.2}$O${}_{3\ensuremath{-}x}$. Reversible, redox-driven topotactic transformations at low temperature and atmospheric pressure indicate that these materials may be of use in electrochemical devices, such as solid oxide fuel cells.