Rates of liquid-solid mass transfer were measured at the inner surface of an annulus by an electrochemical technique which involves measuring the limiting current of the cathodic reduction of K(3)Fe(CN)(6) in aqueous solution with a large excess of NaOH under single and two phase flow using an annulus packed with inert spheres. Variables studied are gas and solution superficial velocities, sphere diameter and physical properties of the solution. For unpacked annulus the two phase mass transfer data are correlated for the conditions: 1253 < Sc < 2778, 28 < Re(L) < 917 and 285 < Re(g) < 4187 by the equation: Sh = 0.58 Sc(0.33)Re(L)(0.62)Re(g)(0.4) For packed annulus, the two phase mass transfer data are correlated for the conditions: 1253 < Sc < 2778, 7 < Re(L) < 8, 4 < Re(g) < 380, 0.8 cm < d(p) < 1.2 cm and annulus diameter of 17 cm by the equation: Sh = 12.69 Sc(0.33)Re(L)(0.1)Re(g)(0.14) The mass transfer enhancement ratio due to the presence of packing spheres ranges from 2.08 to 6.1 depending on ReL, Reg and sphere diameter. Implications of the present work for the design and operation of annular electrochemical reactors suitable for electrochemical synthesis involving gaseous reactants and air pollution control are noted. (C) 2011 Elsevier B.V. All rights reserved.