An important way for the production of biodiesel is the base-catalyzed transesterification of oil or fat with monohydric alcohols. Flow and reaction analysis with respect to the applicability of a micro/millireactor setup to two-phase ethanolysis of waste cooking oil is described. The flow in the micro/millireactor system was examined on the example of a soybean oil/ethanol mixture and compared with a T-mixer. Parallel and slug flow were observed in different reactor sections of the micro/milli-reactor system and evaluated concerning their efficiency. A method for determination of the viscosity of the mixture was conducted and validated. Certain pretreatments were investigated with regard to the suitability of the waste cooking oil for ethanolysis. Relations between biodiesel yields and some oil properties like chain length and acid number were found.