The electronic structure of the heterodimer AlSi is experimentally studied using anion photoelectron spectroscopy. Four low-lying electronic excited states are observed for AlSi. The electron affinity of AlSi is measured to be 1.32 (5) eV, which is lower than that of both Al-2 and Si-2. The electronic structure of AlSi is understood by comparing to that of the known Al-2 and Al-2(-) molecules. The ground state of AlSi is determined to be X (4) Sigma(-) with a vibrational frequency of 400 (50) cm(-1). The four excited states are A (2) Sigma(-), B (2) Delta, C (2) Pi, and D (2) Sigma(+) with excitation energies of 0.23, 0.67, 0.82, and 1.13 eV, respectively. The photoelectron spectra of Al-2(-) are also presented and excited states of the Al-2(-) anion are definitively observed. The electronic structure of AlSi is discussed and compared to that of the homonuclear dimers, Al-2 and Si-2. (C) 1997 American Institute of Physics.