The oxidation of y-MnOOH (manganite) in oxygen and its disproportionation in HNO, lead topotactically to b-MnO,. The oxidation of synthetic a-MnOOH (groutite) in oxygen depends on its cristallite size; finely divided crystals oxidise rapidly to Mn,O, which usually is stable but yields B-MnO, by further oxidation. Larger crystals of disperse synthetic a-MnOOH are topotactically transformed to y-MnO,. In HNO, a-MnOOH disproportionates into y-MnO, and Mn2+. Though strictly topotactical, the reaction cr-MnOOH + y-MnO, is not single-phase as might be expected. The discontinuity in the function : JAHN-TELLER distortion vs. reaction rate, may simply be interpreted as the crosspoint of two different functions attributed to the crystal species a-MnOOH and y-MnO,, respectively. This distortion confirms the presence of MnS+ in manganite and nsutite. The wide variety of possible X ray powder patterns of the phase y-MnO, is explained by the superposition of, (i) cristallite size broadening, (ii) intergrowth structure effects on the profile, and (iii) BRAGG angle shifts due to substitution of part of Mn4+ by Mn3+.