Abstract <p>Layered δ-MnO₂ has been synthesized for the first time using a redox precipitation reaction in the presence of ascorbic acid. A possible mechanism for its formation has been proposed. Subsequent annealing of δ-MnO₂ in air at 500°C leads to the formation of α-MnO₂ with characteristic tunnel structure. The main physicochemical characteristics of the synthesized manganese dioxides have been determined using X-ray powder diffraction and thermogravimetric analysis combined with mass spectroscopy, IR and Raman spectroscopy, high-resolution scanning electron microscopy, and low-temperature nitrogen adsorption. α-MnO₂ sensing properties for various toxic gases have been studied. It has been shown that the sensor based on synthesized oxides can detect H₂S at concentration of 10 ppm with a sensitivity of 35–40%. It has been established that the obtained manganese dioxides have selective sensitivity to 0.2–10 ppm H₂S at 200°C.</p>