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The quantum efficiency and chemical stability of CVD diamond photocathodes has been examined. As-grown or microwave plasma hydrogenated boron-doped diamond films display a quantum photoyield of approximately 0.05% at 190 nm, which degrades gradually as the material is left in ambient atmosphere, due to slow oxidation. Rapid degradation in performance occurs when exposed to atomic or electronically excited oxygen. X-ray photoelectron spectroscopy shows that the yield drops close to zero at around monolayer oxygen coverage, and that the main oxygen species on the surface is hydroxyl or isolated ether linkages.