An efficient practical aerobic oxidation of aldehydes to carboxylic acids using O₂ or air as the oxidant has been developed. The reaction was carried out with a catalytic amount each of Cu(NO₃)₂·3H₂O and TEMPO at room temperature tolerating many useful functional groups such as alkynyl, amino, aryl, etc. Complex molecules and compounds containing multiple aldehyde groups are applicable to this reaction with an excellent selectivity. Gram scale reactions have been easily conducted.