Liquid/liquid electrochemistry has been used to study the transfer of weakly ionised species across the interface between water and 1,2-dichloroethane. It is shown that while transfer of a fully ionised species can be readily used for determination of its diffusion coefficient, transfer of a partially ionised species, such as many common pharmaceutical agents, involves complex ionisation/distribution behaviour, which invalidates the conventional analysis. As a result, the aqueous diffusion coefficient of the transferred species is underestimated by at least one order of magnitude. An alternative method to study the transfer of partially ionised drug molecules employing a rotating liquid/liquid interface is proposed and reported. The alternative approach, which is based on a previously reported rotating diffusion cell approach, employs a lipophilic membrane that stabilises the liquid/liquid interface and allows stirring. This hydrodynamically controlled configuration was successfully applied to transfer of partially ionised drug species, and expected values of the aqueous diffusion coefficient were obtained. (C) 2012 Elsevier B.V. All rights reserved.