Electron spin resonance (ESR) studies are reported for (AlO)-O-16,17, (GaO)-O-16,17, and (InO)-O-16,17 isolated in neon matrices at 4 K. Except for (AlO)-O-16, no previous ESR measurements have been reported for these X (2) Sigma diatomic radicals. The pulsed laser vaporization of the metals in the presence of O-16(2) and O-17(2) produced high quality ESR spectra of these metal oxide radicals whose nuclear hyperfine interactions (A tensors) were fully resolved for both the metal and oxygen nuclei. An analysis of the experimental spin densities in combination with different types of theoretical calculations provided detailed information concerning the electronic structure trends going down this metal oxide group. Increased p-orbital spin density on oxygen was observed for the heavier metal oxide radicals. Nonrelativistic ab initio calculations with an extended basis set and the UB3LYP method reproduced the trends in the isotropic and dipolar hyperfine interactions. All-electron CI calculations, restricted open-shell Hartree-Fock (ROHF) wave functions, and unrestricted Hartree-Fock wave functions gave results very different from experiment and from each other for the isotropic interaction. All calculations were in fair agreement with each other for the dipolar interaction and provided an assignment of the sign for that term. (C) 1997 American Institute of Physics.