In a HF–SbF5 solution at −20∼30 °C under atmospheric pressure, ketones having alkyl groups with five or more carbon atoms underwent the reaction to give corresponding oxo carboxylic acids without any β-scission processes which occur readily in alkyl cations derived by protolysis of alkanes with seven or more carbon atoms. Tertiary C–H bond located at δ or further away from the oxo group in the substrates