Small boron clusters have been shown to be planar from a series of combined photoelectron spectroscopy and theoretical studies. However, a number of boron clusters are quasiplanar, such as B7−and B12−. To elucidate the nature of the nonplanarity in these clusters, we have investigated the electronic structure and chemical bonding of two isoelectronic Al-doped boron clusters, AlB6−and AlB11−. Vibrationally resolved photoelectron spectra were obtained for AlB6−, resulting in an accurate electron affinity (EA) for AlB6of 2.49 ± 0.03 eV. The photoelectron spectra of AlB11−revealed the presence of two isomers with EAs of 2.16 ± 0.03 and 2.33 ± 0.03 eV, respectively. Global minimum structures of both AlB6−and AlB11−were established from unbiased searches and comparison with the experimental data. The global minimum of AlB6−is nearly planar with a central B atom and an AlB5six membered ring, in contrast to that of B7−, which possesses a C2vstructure with a large distortion from planarity. Two nearly degenerate structures were found for AlB11−competing for the global minimum, in agreement with the experimental observation. One of these isomers with the lower EA can be viewed as substituting a peripheral B atom by Al in B12−, which has a bowl shape with a B9outer ring and an out-of-plane inner B3triangle. The second isomer of AlB11−can be viewed as an Al atom interacting with a B11−cluster. Both isomers of AlB11−are perfectly planar. It is shown that Al substitution of a peripheral B atom in B7−and B12−induces planarization by slightly expanding the outer ring due to the larger size of Al.