To avoid the membrane scaling during electrodialytic desalination of sparingly soluble salt solutions, the electrodialysis should be performed in a way that the vast majority of growing clusters is washed out from the module before the crystallization occurs (the residence time is lower than nucleation induction time). Such method requires the determination of residence time distribution (RTD) of the electrodialyzer. While available literature presents only RTD measured at no current conditions, with only water passing through the module, this study presents a method of measuring the RTD on the working electrodialyzer, when the applied current causes ion migration and electroosmotic water flux. The change in concentrate compartment RTD was confirmed for the linear flow velocity range of 0.38-0.84 cm/s and current density range of 0-673 A/m(2). The hydrodynamic conditions inside the module were described using an open-open vessel dispersion model and it was shown that significant longitudinal dispersion is observed (on average D/uL = 0.08). The current density distribution along the electrodialyzer was observed and quantified and the question of velocity distribution along the electrodialyzer was tackled. (C) 2013 Elsevier B.V. All rights reserved.