Based on computational fluid dynamics (CFD), the liquid-liquid mass transfer in a slug flow through circular capillaries was studied. In order to save computational resources and time, the numerical simulations were carried out in a unit cell containing an aqueous slug and an organic one with a moving frame of reference. The flow field within individual slugs and interphase mass transfer with reactions between two consecutive slugs were discussed. Concentration fields were obtained to describe the mass transfer between the liquid phases. In addition, the effects of fluid diffusivity D on mass transfer coefficient k(L)a and the chemical species distribution were studied in detail. In general, the internal circulations inside individual slugs were displayed by the motion paths of mass transfer components. Furthermore, the mass transport process is enhanced by the alkaline hydrolysis reaction as confirmed by the so-called enhancement factor E. Finally, the simulation results showed close agreement with published experimental data.