The homoleptic heterodinuclear copper-nickel carbonyl anions CuNi(CO)(n)(-) (n = 2-4) were generated in a pulsed-laser vaporization source and investigated using photoelectron velocity-map imaging spectroscopy. The electron affinities of CuNi(CO)(2) (2.15 +/- 0.03 eV), CuNi(CO)(3) (2.30 +/- 0.03 eV), and CuNi(CO)(4) (1.90 +/- 0.04 eV) were deduced from the photoelectron spectra. Theoretical calculations at the B3LYP level were carried out to elucidate the structures and the electronic properties of CuNi(CO)(n)(0/1-) (n = 1-4) and to support the experimental observations. Comprehensive comparisons between experiments and calculations suggest that there is a turnover point of the absorption site during the progressive carbonylation process. The carbonyl groups are determined to be preferentially bonded to the nickel atom. When the nickel center satisfies the 18-electron configuration, the copper atom starts to adsorb additional CO molecules. These results will shed light on the bonding mechanisms of the heterometallic carbonyl clusters.