High entropy oxide (HEO) materials display a reversible entropy-driven phase transformation that gives rise to a phase state consisting of the entropic single-phase and controllable volume fractions of a secondary phase, which we call the ‘phase spectrum’. Consolidated HEO samples were prepared with grain sizes spanning several orders of magnitude and were heat treated to demonstrate the dramatic effect that grain size has on controlling the phase spectrum behavior. The volume fraction and morphology of the secondary phase, determined here to be a Cu-rich multicomponent tenorite phase, and the corresponding heat treatment conditions were significantly influenced by the grain size. GRAPHICAL ABSTRACT IMPACT STATEMENT Nanocrystalline high entropy oxides (HEOs) exhibit a wider phase transformation temperature window, as well as changes to the concentration and morphology of the secondary phase, a Cu-rich multicomponent tenorite phase.