D.C. measurements of the conductivity of semi-insulating polar liquids are notoriously difficult to make; almost undetectably small quantities of impurities act as the current carriers, and for many liquids it is not possible to assign a specific value to the conductivity. Very often the current flowing in a conductivity cell falls in a quasi-exponential manner when a constant direct potential is applied across the cell, but it is our experience that the current is just as likely to rise. The current fall has been explained either in terms of the removal of ions from the main body of the liquid or the building of concentration overpotential barriers at the electrodes, while the rise is usually regarded as an “artifact” and its origin is not enquired into.