High-k oxide insulators (Al2O3 and HfO2) have been deposited on a single crystalline hydrogenated diamond (H-diamond) epilayer by an atomic layer deposition technique at temperature as low as 120 °C. Interfacial electronic band structures are characterized by X-ray photoelectron spectroscopy. Based on core-level binding energies and valence band maximum values, valence band offsets are found to be 2.9 ± 0.2 and 2.6 ± 0.2 eV for Al2O3/H-diamond and HfO2/H-diamond heterojunctions, respectively. Band gaps of the Al2O3 and HfO2 have been determined to be 7.2 ± 0.2 and 5.4 ± 0.2 eV by measuring O 1s energy loss spectra, respectively. Both the Al2O3/H-diamond and HfO2/H-diamond heterojunctions are concluded to be type-II staggered band configurations with conduction band offsets of 1.2 ± 0.2 and 2.7 ± 0.2 eV, respectively.