Milli-scale reactors with an integrated microstructure offer a promising scale-up approach for conventional microreactors. This study applies 3D-printed structured porous millireactors to industrially relevant liquid-liquid reactions. The underlying transport mechanisms are identified by quantifying interfacial heat and mass transfer. The structured reactors perform limited in Taylor flow compared to a packed-bed reactor due to limited interfacial mass transfer. However, in stratified flow, their productivity increases significantly at a fraction of the pressure drop of a packed bed.