A process for preparing aliphatic carboxylic acids having 3 to 10 carbon atoms by oxidizing the corresponding aldehydes with oxygen or oxygen-containing gases, characterized in that (i) the oxidation is performed in a microreactor at elevated pressure and with an oxygen excess based on the stoichiometrically required amount of oxygen, and (ii) in that the reaction mixture removed from the microreactor is passed through at least one postreactor without further addition of oxygen, and (iii) wherein the oxidation reaction in the microreactor is performed under such conditions that aldehyde and corresponding peracid formed are present in the reaction mixture removed in a molar ratio within a range of 1:(0.9 to 1.1), preferably of 1:(0.95 to 1.05).