A photoelectron spectroscopic study of ScOn- (n = 1-4) and YOn-(n = 1-5) was carried out at three photon energies : 532, 355, and 266 nm. Vibrationally resolved photoelectron spectra were obtained for ScO- and YO-. The electron affinities of both ScO and YO were measured to be identical (1.35 eV) within the experimental accuracy (+/-0.02 eV). Three low-lying excited states were observed for the monoxides, A＇(2)Delta, A(2)Pi and B(2)Sigma(+). The latter two excited states resulted from two-electron detachment, suggesting unusually strong electron correlation (configuration interaction) effects in the ground state of the anions. The excitation energies of the low-lying states were also found to be similar for the two monoxides except that YO has a smaller vibrational frequency and larger spin-orbit splitting. The A＇(2)Delta states of both ScO and YO show very strong photon energy-dependent detachment cross sections. Four similar photoelectron features were observed for the dioxides with those of YO2- having lower binding energies. A second isomer due to an O-2 complex was also observed for Sc and Y. Broad and featureless spectra were observed for the higher oxides. At least two isomers were present for the higher oxides, one with low and one with high binding energies.