The 100-eV ionization of molecular oxygen has been studied by the (e,2e) electron-electron coincidence method. Ionization to the ground state and five excited states of O2 + has been detected. Of these six electronic states, ionization to three, the X 2&Pgr;g, a 4&Pgr;u, and b 4&Sgr;g -, states, was found to account for approximately 95% of the O2 + production. Relative triple differential cross sections for the production of O2 + in these three states were measured as a function of ejected electron energy, ejected electron angle, and incident electron scattering angle. Ionization to the three higher energy states, B 2&Sgr;g -, 2&Pgr;u, and c 4&Sgr;u -, was found to be much weaker, and it was only possible to estimate the branching ratios to these states. Ionization to the A 2&Pgr;u state, the upper state of the second negative bands of O2 +, was not observed, as this state is only weakly excited and is overlaid by the stronger a 4&Pgr;u state. The branching ratios were found to be 0.540, 0.297, and 0.122 for the X 2&Pgr;g, a 4&Pgr;u, and b 4&Sgr;g - states, respectively. The total branching ratio for the B 2&Sgr;g -, 2&Pgr;u, and c 4&Sgr;u - states was 0.042 (all values¿20%). The branching ratio to the A 2&Pgr;u state was estimated to be <1%. The cross section for the X 2&Pgr;g, a 4&Pgr;u, and b 4&Sgr;g - states were calculated using the previously measured O2 + production cross section for 100-eV electrons on O2. The results were 92.2 (¿20%)¿10-18 cm2, 50.8 (¿20%)¿10-18 cm2, and 22.1 (¿20%)¿10-18 cm2, respectively. The b 4&Sgr;g - state cross section was in satisfactory agreement with the recalculated optical fluorescence data on the O2 + first negative band emission cross section of 37.1 (¿25%)¿10-18 cm2 [McConkey and Woolsey, 1969>.¿ 1997 American Geophysical Union