Abstract Two copper–molybdenum bronzes: Cu y MoO 3− z (0.1 < y < 0.2; typically y ∼ 0.15) and Cu x MoO 3 (0.2 < x < 0.25; typically x ∼ 0.23) were prepared as glistening black polycrystalline materials by the solid state reaction of Cu and MoO 3 at 600 °C under argon in Pt crucibles. Powder XRD showed that the material with global composition ‘0.1Cu·MoO 3’ comprises ∼Cu 0.15MoO 3 and MoO 3; whilst ‘0.2Cu·MoO 3’ comprises ∼Cu 0.15MoO 3 and ∼Cu 0.23MoO 3. DTA performed on ‘0.2Cu·MoO 3’ reveals a reversible solid state phase transition ∼520 °C under argon. Reacting equimolar amounts of Cu 2O and MoO 2 at 600 °C in a Cu crucible under argon yields: Cu 6Mo 5O 18, Cu and MoO 2. A tentative subsolidus Cu–MoO 2–O isothermal (∼25 °C) phase diagram under argon is drawn from these data. Oxidation states of Cu and Mo within this system are discussed.