Experiments are conducted to investigate mass transfer characteristics during flow of two immiscible liquids through glass capillaries of internal diameter 2 mm and 6 mm. Water and toluene -acetic acid mixture are introduced in the test passage by a T mixer and the overall mass transfer co-efficient for acetic acid (10% by volume) diffusing from the organic to the aqueous phase is estimated for the slug, annular, dispersed and inverted dispersed flow patterns. Experiments are conducted for three flow orientations namely horizontal, vertical up and vertical downflow. The data are analysed by considering the prevailing flow pattern and correlating measured values of mass transfer co-efficient (k(ov)a) and solute transferred per unit pressure gradient with input parameters. It is seen that (k(ov)a) and solute transferred per unit pressure gradient is highest in case of slug flow as compared to other flow patterns. Further, (k(ov)a) is a function of both tube diameter and flow orientation, being higher in the smaller tube diameter and in downflow orientation of both the capillaries. The relationship between (k(ov)a) and energy dissipation factor is observed to be a function of phase velocity and flow pattern. Based On the experiments, the study proposes that an efficient co-current tubular contactor should be trumpet shaped with short horizontal sections connecting long vertical up and downflow sections. (C) 2014 Elsevier Ltd. All rights reserved.