Elucidating the chemisorption properties of CO on gold clusters is essential to understanding the catalytic mechanisms of gold nanoparticles. Gold hexamer Au-6 is a highly stable cluster, known to possess a D-3h triangular ground state structure with an extremely large HOMO-LUMO gap. Here we report a photoelectron spectroscopy (PES) and quasi-relativistic density functional theory (DFT) study of Au-6-CO complexes, Au-6(CO)(n)(-) and Au-6(CO)(n) (n = 0-3). CO chemisorption on Au-6 is observed to be highly unusual. While the electron donor capability of CO is known to decrease the electron binding energies of Au-m(CO)(n)(-) complexes, CO chemisorption on Au6 is observed to have very little effect on the electron binding energies of the first PES band of Au-6(CO)(n)(-) (n = 1-3). Extensive DFT calculations show that the first three CO successively chemisorb to the three apex sites of the D-3h Au-6. It is shown that the LUMO of the Au-6-CO complexes is located in the inner triangle. Thus CO chemisorption on the apex sites (outer triangle) has little effect on this orbital, resulting in the roughly constant electron binding energies for the first PES band in Au-6(CO)(n)(-) (n = 0-3). Detailed molecular orbital analyses lead to decisive information about chemisorption interactions between CO and a model Au cluster.