The Al₃O₂, Al₃O₃, Al₄O_x, Al₅O_x (x = 3–5), Al₆O₅, and Al₇O₅ clusters are studied using negative ion photoelectron spectroscopy. At 266 nm (4.661 eV) laser photodetachment wavelength the spectra of Al₃O₂ and Al₃O₃ present vibrationally resolved features. They show three electronic transitions, due to two different isomers. From Franck–Condon simulations of the Al₃O₂ and Al₃O₃ photoelectron spectra, several vibrational frequencies together with the normal coordinate changes were derived. We obtained approximate electron affinities for Al₃O₂ and Al₃O₃ using the Gaussian 2 model, and calculated isomerization energies for both the anionic and neutral geometries and the experimental adiabatic detachment energies (ADE) of bands X′ and X. The larger aluminum oxide clusters present several structureless bands which likely also result from multiple isomers. The ADEs for the larger clusters increase with size within a cluster series, with the exception of Al₅O₅.