High-resolution threshold photoelectron spectroscopy has been used to study O-2 in the 24-50 eV photon energy region, where several broad bands with superimposed fine peaks are observed. Theoretical calculations on the basis of the state-averaged complete-active-space self-consistent-field method followed by the second order configuration interaction method reproduce well the general band features of the threshold photoelectron spectrum, enabling us to assign the bands to inner-valence O-2(+) states. Dissociation products formed from the inner-valence O-2(+) states have been investigated by threshold photoelectron-photoion coincidence spectroscopy and their dissociation dynamics discussed with reference to the theoretical potential energy curves. (C) 2003 American Institute of Physics.