Photoelectron spectra of a series of Al3Oy- clusters (y=0-5) are presented at several photon energies: 532, 355, 266, and 193 nm. The electron affinities and low-lying electronic states of the Al3Oy clusters are reported. The photoelectron spectra clearly reveal a sequential oxidation behavior and how the electronic structure of the clusters evolves from that of a metal cluster at Al, to that of a complete oxide cluster at Al3O5: Two valence electrons of Al-3 are observed to be transferred to each additional O atom until Al3O5, where all the nine valence electrons of Al-3 are transferred to the five O atoms. The anion, Al3O5, which can be viewed as (Al3+)(3)(O2-)(5), is found to be a closed shell cluster, yielding an extremely high electron affinity for Al3O5 (4.92 eV). The electron affinities of the remaining clusters are: 1.90 (Al-3), 1.57 eV (Al3O), 2.18 eV (Al3O2), 2.80 eV (Al3O3), and 3.58 eV (Al3O3) An electronic excited state of Al; is also observed at 0.40 eV above the Al; ground state. Isomers are observed for all the oxide clusters with lower electron affinities; Particularly, vibrational structures are observed for the two isomers of Al3O3-, as well as a photoisomerization process between the two isomers. The structure and bonding of the oxide clusters are discussed based on the experimental data and the known structures for Al-3 and Al3O.