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The infrared absorption spectra of α‐, β‐, γ‐ and δ‐ MnO2 prepared by different methods, were measured and discussed. It was found that the ratio of absorbancies of the bands at 595 ± 20 cm−1 und 535 ± 20 cm−1 (φ) is related to the amorphous content. Each preparation is characterized by a certain value of φ. The mode of change of φ as a result of heating the different preparations is discussed. Infrared spectra help in differentiating α‐, γ and δ‐MnO2 on the one hand, and β‐MnO2 on the other, and in indicating the transformation of γ‐into β‐MnO2 by heating the former at 450 °C.