The mixed dialkyl oxalate (mDAO) system can be industrially synthesized inexpensively from CO with methanol, ethanol, and mixed alcohols, which are widely available from CO-rich industrial waste gases and syngas. In this work, a series of mDAOs were synthesized through solvent-free partial transesterification of dimethyl oxalate (DMO) or diethyl oxalate (DEO) with higher alcohols. The physical and chemical properties of various mDAOs and different oxygenated fuels were compared, and the viability of the mDAO as an oxygenated fuel additive was tested. The experimental data indicated that the mixed methyl ethyl oxalate (mMEO) is miscible with gasoline in any proportion at a temperature above 263 K. The mixed methyl butyl oxalate (mMBO) and mixed ethyl butyl oxalates (mEBO) show better solubility in diesel than mMEO. The research octane number (RON) of the mMEO/gasoline composite fuel increased with the increase of the mMEO concentration. The flash point, kinematic viscosity, and cetane number of the diesel mixed with a content of 10-20% mMBO meet the requirements of the China national standard. Our results indicate that mDAO can be used as a promising oxygenated fuel additive for gasoline and diesel fuels.