Unintentionally doped Al0.67 Ga0.33 N epilayers were grown on AlNsapphire templates by metalorganic chemical vapor deposition. Optimized undoped Al0.67 Ga0.33 N epilayers exhibited an n -type conductivity as confirmed by Hall-effect measurement with a room-temperature resistivity of about 85 Ω cm. Variable temperature Hall-effect measurement revealed the existence of a shallow donor level with activation energy of about 90 meV. The photoluminescence (PL) spectra exhibited an emission peak at 4.13 eV (4.06 eV) related to an impurity transition at 10 K (300 K). Temperature dependent PL measurement also confirmed the existence of a shallow donor with comparable activation energy as that obtained by Hall-effect measurement. Isolated oxygen impurities are believed to be a strong candidate of the donor that remains as a shallow state in Alx Ga1-x N up to x∼0.7. Compensating defects and the nature of the O donor in Al0.67 Ga0.33 N epilayers are also discussed.