The electronic structure of Cu(2)O and CuO thin films grown on Cu(110) was characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The various oxidation states, Cu(0), Cu(+), and Cu(2+), were unambiguously identified and characterized from their XPS and XAS spectra. We show that a clean and stoichiometric surface of CuO requires special environmental conditions to prevent loss of oxygen and contamination by background water. First-principles density functional theory XAS simulations of the oxygen K edge provide understanding of the core to valence transitions in Cu(+) and Cu(2+). A novel method to reference x-ray absorption energies based on the energies of isolated atoms is presented.