This work reports Computational Fluid Dynamics (CFD) modelling of mass transfer in liquid–liquid Core- Annular Flow (CAF) in a microchannel. The model solves Navier-Stokes and convection–diffusion equations after incorporating interfacial boundary conditions of velocity, shear stress, flux and concentration jump. The model has been validated with the analytical and previously reported results. The validated model has been used to understand and quantify the dependence of concentrations boundary layer thicknesses on the core side and annulus side on Reynolds number, Schmidt number, dimensionless length and distribution coefficient. Furthermore, the dependence of local mass transfer coefficients on the core and annulus side on diffusivity and concentration boundary layer thicknesses is quantified and compared with the predictions of film theory and penetration theory. Finally, the results obtained from the CFD model have been used to obtain the correlations of Sherwood number on the core side and the annulus side.