Open-system, continuous-titration experiments have been done in which a slow flux of ∼0.02 molar solution of Mn²⁺ chloride, nitrate, or perchlorate with Cu²⁺ or Ni²⁺ in lesser concentrations was introduced into an aerated reactor solution held at constant temperature and at constant pH by a pH-stat titrator that added dilute NaOH. The resulting mixtures of metal oxyhydroxides and their native solutions were aged for periods as long as 2 1/2 years. Fresh and aged precipitates were characterized by chemical analysis, oxidation state determinations, X-ray and electron diffraction, and electron microscopy. The precipitates can be described as mixtures of oxide and oxyhydroxide species, using concepts of equilibrium and nonequilibrium chemical thermodynamics. The metal-ion content of the aged precipitates in systems that contained copper is distributed among three principal components. One of these is a mixed oxide Cu₂Mn₃O₈ in which all Mn is in the 4+ oxidation state. A major component in all precipitates is feitknechtite, βMnOOH. These forms are supplemented by CuO or by birnessite or ramsdellite forms of MnO₂ where stoichiometry and thermodynamic calculations predict them. In systems that contained nickel and manganese, identifiable components included βMnOOH, Ni(OH)₂, and the same two forms of MnO₂. The oxidation number of the precipitated manganese increased during aging, and the pH of the supernatant solution decreased. The maximum Mn oxidation number observed was 3.55 in an Mn + Cu precipitate aged for 18 months. Concentrations of Cu²⁺ and Ni²⁺ generally decreased to values substantially below those predicted by oxide or hydroxide equilibrium. Scavenging effects of this type are common in natural aqueous systems.