The electrochemical technique with the ferri-ferrocyanide system has been used to obtain transient mass transfer coefficients for smooth pipes. In contrast to the analytical solution of the corresponding heat transfer problem, two distinct transient periods are observed. It is proposed that the first is controlled by chemical reaction kinetics at the surface and during that period the boundary condition of zero ferricyanide concentration at the cathode surface is approached. The diffusion-controlled period follows in agreement with the heat transfer solution. The transfer rate for laminar flow is then proportional to (D/t)(1/2), in accordance with Higbie's penetration theory. By analogy, the data also confirm the analytical heat transfer results. In turbulent flow, the mass transfer rate during the first transient period is the same as in laminar flow; during the second, it is proportional to Re-1/4(D/t)(1/2), higher than in laminar flow. Therefore, the first transient period is longer and the second shorter than for laminar flow conditions.