The kinetics of the exchange between MnO or Mn3O4 and Co2+, Cu2+, Ni2+, or Zn2+ ions in solution, was determined by measuring the γ-activity of56Mn acquired by the solution after shaking with the neutron irradiated solid. The results indicated a fast exchange followed
by a slower apparently diffusion-controlled exchange. The exchange capacity increased in the series: Co2+<Ni2+<Cu2+=Zn2+ for MnO and Ni2+<Co2+<Zn2+<Cu2+ for Mn3O4. The trends could not be satisfactorily explained by the ionic radii or crystal field stabilisation-energies. In the case
of MnO, the results were discussed in terms of the estimated standard enthalpy change of the exchange reaction.