The photooxygenation of alpha-terpinene was investigated in a falling film microreactor. The formation of micro-films and the flow operation allowed for a safe and controllable production of the bioactive endoperoxide ascaridole. Stepwise investigation of the process parameters resulted in the selective formation of ascaridole of up to 89% and a productivity of 2.5-3.2 mol L-1 h(-1), respectively.