Abstract <p>This study investigates the structure and phase transformations of manganese oxides synthesized by the sol–gel process in the Mn–O–(Co) system in the presence of Sr²⁺ ions and subjected to thermal modification and acid protonation. The goal of the study was to establish the relationship between the phase composition, textural characteristics, and reduction properties of Sr-containing manganese oxides. The study was performed using X-ray diffraction, Raman spectroscopy, low-temperature nitrogen adsorption, and hydrogen temperature-programmed reduction. It was shown that heat treatment at 150°C stabilizes a metastable layered vernadite/birnessite-like framework, whereas heating to 360°C leads to the formation of tunnel and spinel MnO₂ phases. Acid protonation is accompanied by leaching of Sr²⁺ ions and an increase in the specific surface area up to ∼270 m²/g. It was established that cobalt doping (5 mol&#xa0;% relative to Mn), in combination with high-temperature treatment and subsequent protonation, decreases the reduction temperature and increases the fraction of easily reducible oxygen species. The obtained results are of interest for the development of oxide materials with enhanced redox activity.</p>