The 351 nm photoelectron spectra of Mo- and MoO- have been measured. The electron affinity of atomic molybdenum is 0.748(2) eV and that of molybdenum monoxide is 1.290(6) eV. The term energies of several MoO electronic states not previously observed are obtained and compared with ab initio predictions. The ground state of MoO is confirmed to have (5) Pi symmetry and the term energy of the (3) Pi excited state, 10 179(20) cm(-1), closely matches calculations. The ground state of MoO- is a (4) Pi State with a vibrational frequency of 810(40) cm(-1). The first excited state of molybdenum monoxide is tentatively assigned as a (3) Delta state with T-0 = 621(50) cm(-1). At least one state, possibly a (5) Sigma(-) state, lies 8000(500) cm(-1) above the ground state, and a (5) Sigma(+) State is observed at 11 590(60) cm(-1) above the ground state. The separations of spin-orbit levels for the MoO X (5) Pi, (3) Pi, and (3) Delta states are 169(30), 410(20), and -720(20) cm(-1) respectively. The vibrational frequencies of the (3) Pi and (3) Delta states are found to be 600(20) and 1000(20) cm(-1), respectively. These observations give new insight into the Mo-O bond.