Immiscible liquid-liquid flow patterns and mass transfer were investigated in circular PTFE capillaries, with toluene-sulfuric acid, toluene-water and ethyl acetate-water systems. By comparing the slug flow operation range at different temperatures and capillary diameters, the results revealed the roles of inertia and viscous force in the transition from slug flow to droplet flow, and from slug flow to annular flow. A universal flow map based on composite terms of Ca(C)Re(C)(0.5)andCa(D)(0.7)Re(D)(0.5) was proposed to represent the competition between interfacial tension and the inertia/viscous force, which can excellently predict experimental data and literature results with fluid viscosity ranging from 0.85 to 1200 mPa.s. Additionally, the effect of temperature on the dispersed phase slug velocity, specific surface area and mass transfer was investigated and discussed, providing incremental understanding on the flow hydrodynamics and better guidance for optimized reactor design.