A continuous flow method for the selective oxidation of secondary alcohols is reported. The method is based on an Oppenauer-type ruthenium-catalyzed hydrogen-transfer process that uses acetone as both solvent and oxidant. The process utilizes a low loading (1 mol%) of the commercially available ruthenium catalyst [Ru(p-cymene)Cl₂]₂and triethylamine as a base and can be successfully applied to a range of different substrates, with a good level of functional group tolerance.