The protein cage of the 12-subunit ferritin-like protein from Listeria innocua has been utilized as a size and shape constrained reaction environment for the synthesis of two cobalt oxide minerals, Co3O4 and Co(O)OH. Reaction of Co(II) with H2O2 at pH 8.5 under either elevated temperature (65 degreesC) or ambient temperature (23 degreesC) resulted in the formation of cobalt oxide nanoparticles encapsulated within the protein cage. At elevated temperatures, Co3O4 was formed while at lower temperature the oxyhydroxide Co(O)OH was found. Mineral particles, commensurate in size with the internal dimensions of the protein (5 nm), were imaged by transmission electron microscopy and shown to be surrounded by the intact protein cage. The minerals were investigated by electron diffraction and revealed a crystalline Co3O4 phase and an amorphous Co(O)OH phase. Further investigation of these composite materials using size exclusion chromatography, gel electrophoresis, dynamic light scattering, and potential measurements indicated that the mineral was encapsulated within the protein cage giving rise to properties of both the mineral and protein components.