Mass transfer was studied at the external electrode of a cylindrical electrochemical reactor with decaying swirling flow using a single phase electrolyte and also a dispersion of a gas phase in the solution. The local mass-transfer coefficients were measured with a segmented cathode using the reduction of ferricyanide as test reaction. Two strategies were used to introduce the gas phase. In the first case the gas was fed by three holes symmetrically drilled at the bottom of the central anode tube and in the second one by means of a tee fitting at the reactor inlet. The best performance was obtained with the single phase electrolyte giving 0.78 as the exponent of the Reynolds number and the mass-transfer distribution along the reactor length is more uniform than that reported for a parallel plate electrochemical reactor under laminar flow conditions. The mass-transfer enhancement factor related to an annular duct with axial developing flow ranges from 8 to 11. This high value is the consequence of two factors: (i) the intensive forced convection produced by the swirling flow at the outer electrode and (ii) the sudden expansion of the flow at the entrance due to the small tangential inlet. The fed of a gas phase in the reactor has scarce influence on the mass-transfer performance. A comparison of mass-transfer results with previous studies is made.