Partial reduction of bulk CuAlO2 results in hierarchical structures wherein there are copper-alumina regions with widely differing morphologies and scale. At the finest level, the distribution of the 2 phases is at the nanoscale. By means of atomic resolution STEM and electron diffraction, the nanocomposite regions were shown to consist of a dense array of metallic copper platelets dispersed in a matrix of -Al2O3. The copper nanoplatelets were single crystal, and they all exhibited the same orientation relationship with the matrix, namely [110](cu)//[010](theta-Al2O3). (III)(cu)//(4 0 2)(theta-Al2O3). it was shown that the 2-phase regions where the copper exhibited a significantly coarser, globular morphology, resulted from discontinuous coarsening. Interestingly, a change in the matrix phase from theta- to delta-alumina was also observed as a result of the coarsening reaction. It is believed that in the nanocomposite regions, the theta-alumina phase was stabilized by the lower interfacial energy between the copper (110) platelets and the matrix.