Beta MnOOH is precipitated preferentially (with respect to Mn₃O₄) at temperatures near O°C when Mn²⁺ is oxidized in aerated aqueous solutions. Upon aging in solutions open to the atmosphere a slurry of βMnOOH tends to disproportionate to form MnO₂ and Mn²⁺. In such aged solutions, Mn²⁺ and H⁺ activities can be constant, and both the oxidation reaction Mn²⁺+¼O₂(aq) + 3/2H₂O → βMnOOH (c) + 2H₊ and the disproportionate reaction 2βMnOOH (c) + 2H⁺ → MnO₂(c) + Mn²⁺ + 2H₂O can have positive reaction affinities. It is not possible for both reactions to be in thermodynamic equilibrium in the same system unless oxygen is almost completely absent. A value for ΔG_f⁰ of −129.8±0.6 kcal/mol was obtained for βMnOOH from experimental data by assuming that the reaction affinity for the oxidation reaction is equal to that for the disproportionation. A value for ΔG_f⁰ for βMnOOH of −129.8±0.5 kcal/mol was determined by measuring the redox potentials for the postulated half-reaction MnO₂ (c) + H⁺ + e⁻ → βMnOOH (c) at 0°, 5°, and 15°C and extrapolating to 25°C. Both these values are consistent with laboratory observations that βMnOOH is less stable than γMnOOH or Mn₃O₄ at 25°C. Analytical data for manganese-depositing springwater samples are consistent with a nonequilibrium model involving disproportionation of either βMnOOH or Mn₃O₄.