We present the photoelectron spectrum of CsO-, recorded using 2.540 eV photons. This spectrum provides a direct measurement of the X(2) Sigma(+)A(2)II energy splitting in CsO, which is found to be 0.135+/-0.025 eV. This work also establishes that the ground state of CsO- is (1) Sigma(+). In addition, the adiabatic electron affinity of CsO is found to be 0.273+/-0.012 eV, while the D-o value for the X(1) Sigma(+) state of CsO- (with respect to Cs+O-) is found to be 1.84+/-0.15 eV. Molecular parameter estimates for CsO- are also extracted from the spectrum.