Rates of solid-liquid mass transfer were measured at the inner surface of an annular duct by the electrochemical technique under developing flow conditions. Variables studied were physical properties of the solution, velocity, length of the annulus, inlet port diameter and surface roughness. Inlet port diameter was found to have no effect on the rare of mass transfer. For smooth annuli the laminar flow data fit the equation : Sh = 1.029Sc(0.33)Re(0.55)(L/d)(-0.472) The turbulent flow data fit the equation : Sh = 0.095Sc(0.33)Re(0.85)(L/d)(-0.472) Surface roughness in the form of V-threads normal to the how was found to have a negligible effect on the rate of mass transfer in the laminar flow region while in the turbulent region the data fit the equation : Sh = 0.167Sc(0.33)Re(L/d)(-0.472)(e/d)(0.33) Under the present conditions, where the dimensionless roughness height e(+) lies between 0.5 and 22, the rate of mass transfer was found to increase by an amount ranging from 10% to 200% depending on e(+).