Process intensification via miniaturization has become an attractive research field for industry and R&D especially for the production of fine chemicals and pharmaceuticals due to enhanced mass and heat transport. Fabrication of helically coiled tubular devices (HCTDs) in micro-scale can further enhance the mass and heat transfer due to the formation of secondary flow profile at laminar flow. Liquid–liquid (L–L) mass transfer performance of different microstructured HCTDs were investigated for slug flow patterns. A complete microextraction system was constructed and characterized including a T-junction (T-mixer) for slug flow generation, HCTDs as residence time units (RTUs), and a continuously working in-line phase splitter for an instantaneous phase separation. RTUs were fabricated by using fluorinated ethylene propylene (FEP) tubes (ID = 1 mm). EFCE test system, namely, n-butyl acetate/acetone/water system was chosen as an extraction system for the mass transfer characterization. The total volumetric flow rate and the volumetric flow ratio of aqueous to organic phase were varied in the range of 1–8 mL min−1 and 0.5–2.0, respectively. Effects of residence time, flow ratio, and the generation of secondary flow profile, i.e. Dean vortices on L–L mass transfer were investigated and results were compared with straight capillaries. Results revealed that a certain type of HCTD, i.e. coiled flow inverter (CFI) offers higher extraction efficiencies up to 20% in comparison to straight capillaries at constant residence times. Additionally, it was found that for slug flow patterns, Dean vortices provide enhanced L–L mass transfer compared to Taylor vortices that occur in straight capillaries. A complete, continuously operated microextraction system was developed for single-stage applications, where very small liquid hold-ups and longer residence times are required due to slower mass transfer rates.