This study investigates ionic transport and selectivity in a thin-film composite nanofiltration membrane (NF270) operated in pressure-driven and electrodialysis processes. The perm-selectivity and permeabilities of anions (SO42- and NO3-, SO42- and Cl-, NO3- and Cl-) and cations (Mg2+ and Na+) were studied in an electrodialysis cell operated with NF membrane (ED-NF) at varied trans-membrane potentials (0.5-2 V) and in conventional nanofiltration process. The permeabilities of all ions in the electrochemical process were one to four orders of magnitude lower than permeabilities reported for the pressure-driven NF operated with the same membrane. In the pressure-driven process, a high selectivity of mono-to di-valent ions was obtained, with anions selectivity higher than cations selectivity (highest Na+ to Mg2+ selectivity value of 4.0 vs. lowest Cl- to SO42- selectivity value of 11.6). In contrast, in ED a low perm-selectivity of mono-to di-valent ions was obtained (1.1-1.2 for Na+/Mg2+, and 2.4-3.4 for Cl-/SO42-). Our findings show that: (1) ionic transport in thin film composite membranes operated in electrochemical processes cannot be described by conventional ion exclusion models applied to pressure-driven processes, such as Born or Donnan exclusion mechanisms; and (2) a different, currently unknown, mechanism dominates the ion separation.