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Wide-bandgap semiconductors have unique electron emission properties by virtue of having high-lying conduction bands. Among these, diamond stands out because of its chemical stability, allowing it to serve as a solid-state electron source in vacuum and non-vacuum environments, including water. However, the underlying mechanisms of electron emission are not well understood. Here, we report investigations of the mechanisms of electron emission from H-terminated and oxidized surfaces of single-crystal boron-doped diamond(111) in vacuum and in water using both sub-bandgap (4.75 eV and 3.05 eV) and above-bandgap (21.2 eV) excitation. Energy-resolved photoemission spectra in vacuum using different incident photon energies reveal two distinct energy distributions, reflecting different emission pathways. While oxidation greatly reduces electron emission into vacuum using both sub-bandgap and above-bandgap sources, facile electron emission into water persists on the oxidized samples using sub-bandgap excitation and is directly observed through transient optical absorption measurements using sub-bandgap excitation. Low-energy inverse photoemission spectroscopy shows that oxidation leads to broad distribution of surface states throughout the diamond bandgap. Our studies highlight