Solid solutions involving natural Mn³⁺O(OH) polymorphs, groutite, manganite, and feitknechtite are characterized and discussed based on original and literature data on the chemical composition, powder and single-crystal X-ray diffraction, and middle-range IR absorption spectra of these minerals. It is shown that manganite forms two kinds of solid-solution series, in which intermediate members have the general formulae (i) (Mn⁴⁺, Mn³⁺)O(OH,O), with pyrolusite as the Mn⁴⁺O2 end-member, and (ii) (Mn³⁺, M²⁺)O(OH, H2O), where M = Mn or Zn. In Zn-substituted man-ganite from Kapova Cave, South Urals, Russia, the Zn²⁺:Mn³⁺ ratio reaches 1:1 (the substitution of Mn³⁺ with Zn²⁺ is accompanied by the coupled substitution of OH⁻ with H2O). Groutite forms solid-solution series with ramsdellite Mn⁴⁺O2. In addition, the incorporation of OH⁻ anions in the 1 × 2 tunnels of ramsdellite is possible. Feitknechtite is considered to be isostructural with (or structurally related to) the compounds (M²⁺, Mn³⁺)(OH, O)2 (M = Mn, Zn) with a pyrochroite-related layered structure.