The chemical structure and bonding of the hypermetallic Al5C and Al5C- species have been studied by photoelectron spectroscopy and ab initio calculations. Both Al5C (C-2v, (2)A(1)) and Al5C- (C-2v, (1)A(1)) are found to have planar structures that can be related to that of the planar square Al4C- by adding one Al+ ion or one Al atom to an edge of the square. The planarity of Al5C and Al5C- can be explained in terms of the structure of their highest occupied molecular orbitals which are ligand five-center one- or two-electron bonding MO, respectively, similar to the orbital responsible for the planarity of Al4C-. Four peaks were observed in the photoelectron spectra of Al5C- with vertical binding energies of 2.67, 2.91, 3.19, and 4.14 eV which compare well with the 2.68, 2.96, 3.27, and 4.35 eV calculated by the Green function method [OVGF/6-311+G(2df )]. The excellent agreement between the calculated and experimental electron affinity and excitation energies allow us to completely elucidate the geometrical and electronic structures of Al5C- and suggest the most likely structure for the Al5C molecule.