High energy (e.g., ultrasonic) mixing of a hydrocarbon feedstock and reactants comprised of an oxidation source, acid, and optional catalyst yields a liquid hydrocarbon product having increased cetane number. Ultrasonic mixing creates cavitation, which involves formation and violent collapse of micron-sized bubbles, which greatly increases reactivity of the reactants. Cavitation substantially increases cetane number compared to reactions carried out using conventional mixing processes, such as simple mechanical stirring. An aqueous mixture comprising water and acid can be pretreated with ozone or other oxidizer using ultrasonic cavitation prior to reacting the pretreated mixture with a hydrocarbon feedstock to promote cetane-increasing reactions. Controlling temperature inside the reactor promotes beneficial cetane-increasing reactions while minimizing formation of water-soluble sulfones.