To　examine　the　scalability　of　a　microreactor　to　the　milli-scale,　the　liquid–liquid　two-phase　flow　and　mass　transfer　in　a　capillary　reactor　system　were　investigated.　The　flow　pattern　gradually　transformed　from　a　surface-force-controlled　flow　into　a　gravity-force-controlled　flow　with　an　increase　of　hydrodynamic　diameter.　In　the　T-mixer,　the　volumetric　mass-transfer　coefficient　decreased　and　increased　with　an　increase　in　the　capillary　diameter　at　low　and　high　superficial　liquid　velocities,　respectively.　Because　of　the　different　mass-transfer　mechanisms　in　the　T-mixer　and　capillary,　this　scale-up　effect　became　weaker　in　the　capillary　reactor　system,　particularly　at　a　high　superficial　liquid　velocity.　A　minimum　18%　of　mass　was　transferred　in　the　T-mixer　even　at　the　milli-scale,　suggesting　the　significant　contribution　of　the　T-mixer　to　mass　transfer　in　the　capillary　reactor　system.　Empirical　equations　for　predicting　the　volumetric　mass-transfer　coefficient　of　the　T-mixer　and　the　capillary　reactor　system　in　the　hydrodynamic　diameter　range　of　0.8–3.0　mm　were　formulated.　©　2020　Elsevier　Ltd