The concentration profile in a laminar flow of the diluent of a parallel-plate electrodialysis stack is derived by assuming operation at the limiting current density. The Nusselt mass transfer number is found to depend on the Reynolds number, the Schmidt number and the dimensionless hydraulic equivalent diameter according to = 3.7 . (Re . Sc . de/L)'/3. An empirical correlation of the Reynolds number, the Schmidt number, and the shape factor of the electrodialysis cell, with the ionic mass transfer rate in ion exchange membrane electrodialysis is obtained by measuring the limitin current densities at various conditions. The resulting empirical mass-transfer correlation,