An @[email protected] between phases containing free charged components which are usually accumulated or depleted in the @[email protected] thus giving rise to net charges on the phase. This definition includes the special case when the net charge on each of the phases reduces to zero. Charged components may or may not cross the @[email protected] between two phases, so that interphases may be divided into the limiting types unpolarizable and polarizable, respectively. @[email protected] are those for which the exchange of common charged particles between the phases proceeds unhindered. @[email protected] are those for which there are no common components between the phases or the exchange of these is hindered. This condition may arise as a result of the equilibrium conditions or from the kinetics of charge transfer and leads to an @[email protected] which is impermeable to @[email protected] @[email protected] may approach more or less well one of these idealized cases. Polarisibility or non-polarisibility is not an absolute property of an @[email protected] but depends on a number of conditions, e.g. time scale of the experiment.