Handling hazardous multiphase reactions in flow brings not only safety advantages but also significantly improved performance, due to better mass transfer characteristics. In this paper, we present a continuous microreactor setup, capable of performing olefin autoxidations with O-2, under solvent-free and catalyst-free conditions. Owing to the transparent reactor design, consumption of O-2 can be visually followed and exhaustion of the gas bubbles marks a clear end point along the channel length coordinate. Tracking the position of this end point enables measuring effective rate constants. The developed system was calibrated using the well-studied beta-pinene substrate, and was subsequently applied to the synthetically interesting transformation of (+)-valencene to (+)-nootkatone. For the latter, a space-time yield was obtained that is at least 3 orders of magnitude larger than that realized with established biotechnology approaches.