One　of　the　important　strategies　for　the　scale-up　of　microreactors　is　sizing-up,　which　is　conducted　by　increasing　the　hydrodynamic　diameter　of　microreactors.　However,　the　interphase　mass　transfer　deteriorates　seriously　in　the　sizing-up.　This　work　aimed　to　probe　the　possibility　of　adding　an　inert　gas　phase　to　offset　the　adverse　effect　of　microreactor　sizing-up　on　the　mass　transfer　between　two　immiscible　liquid　phases.　Using　a　high-speed　camera,　four　flow　patterns　were　observed　in　three　capillaries　with　their　diameters　ranging　from　0.8　to　3.0　mm.　Empirical　equations　were　given　to　describe　the　flow-pattern　transitions.　The　influencing　mechanism　of　the　capillary　diameter　on　the　liquid–liquid　mass　transfer　was　analyzed　by　taking　the　effect　of　adding　the　inert　gas　phase　into　account.　Finally,　the　evaluation　of　the　energy　consumption　suggested　that　adding　an　inert　gas　phase　to　agitate　the　flow　could　utilize　the　input　energy　more　efficiently　to　intensify　the　liquid–liquid　mass　transfer　in　the　microchannel　with　a　larger　hydrodynamic　diameter.　Therefore,　the　method　of　inert　gas　agitation　is　a　meritorious　assistive　technology　in　the　sizing-up　of　microreactors.　©　2024　Elsevier　Inc.