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The effects of isoelectronic substitution on the electronic and structural properties of gold clusters are investigated in the critical size range of the two-dimensional (2D)-three-dimensional (3D) structural transition (MAun −, n=8–11; M=Ag,Cu) using photoelectron spectroscopy and density functional calculations. Photoelectron spectra of MAun − are found to be similar to those of the bare gold clusters Aun+1 − , indicating that substitution of a Au atom by a Ag or Cu atom does not significantly alter the geometric and electronic structures of the clusters. The only exception occurs at n=10, where very different spectra are observed for MAu10 − from Au11 −, suggesting a major structural change in the doped clusters. Our calculations confirm that MAu8 − − possesses the same structure as Au9 − with Ag or Cu simply replacing one Au atom in its C2v planar global minimum structure. Two close-lying substitution isomers are observed, one involves the replacement of a center Au atom and another one involves an edge site. For Au10 − we identify three coexisting low-lying planar isomers along with the D3h global minimum. The coexistence of so many low-lying isomers for the small-sized gold cluster Au10 − is quite unprecedented. Similar planar structures and isomeric forms are observed for the doped MAu9 − clusters. Although the global minimum of Au11 − is planar, our calculations suggest that only simulated spectra of 3D structures agree with the observed spectra for MAu10 −. For MAu11 −, only a 3D isomer is observed, in contrast to Au12 − which is the critical size for the 2D-3D structural transition with both the 2D and 3D isomers coexisting. The current work shows that structural perturbations due to even isoelectronic substitution of a single Au atom shift the 2D to 3D structural transition of gold clusters to a smaller size.