Au_144−xCu_x(SR)₆₀ alloy nanomolecules were synthesized and characterized by using ESI-MS to atomic precision. A maximum of 23 copper atoms replace the gold atoms. When the number of copper atoms is higher than eight, a surface plasmon-like peak appears at ∼520 nm. Based on the fundamental elemental properties of Au and Cu, we predict a mixed atomic ordering and incorporation of Cu atoms into the Au₁₂ and Au₄₂ shells of the proposed icosahedral structure model of the 144-atom nanomolecule.