The electronic and molecular structure of a series of nonstoichiometric pentaatomic species, Al3X2- (X = C,Si,Ge) and their corresponding neutrals. have been studied by photoelectron spectroscopy and ab initio calculations. Well-resolved photodetachment transitions were observed for all three species and were compared with the calculations. Al3C2-was found to have a C-2v((1)A(1)) global minimum with an Al-ion coordinated to the CC triple bond in a quasi-linear AlCCAl molecule. Al3Si2- and Al3Ge2- were found to be different from the isoelectronic Al3C2- anion. They each have an almost planar trapezium global minimum structure with several low-lying isomers. The quasi-planarity of Al3Si2- and Al3Ge2- was understood by comparison with the previously known SiAl4-planar molecule.