Ag/AgCl reference electrodes fabricated from pulled quartz tubes with orifice radii of 20 nm to 20 µm were characterized in KCl solutions of different concentrations by cyclic voltammetry. Linear current-voltage (i-V) dependence was observed with micropipet electrodes (with micrometer-sized tips) with the same concentration (0.01-1 M) of KCl inside and outside the pipet, but current rectification was found at nanopipet electrodes at KCl concentrations of e0.1 M (with nanometer-sized tips). This is attributed to formation of a diffuse electrical double layer within the tip orifice. The effects of electrode size, electrolyte concentration, and solution pH on the nonlinear i-V behavior of these electrodes were investigated. A model for the observed behavior shows the rectification to be caused by the permselectivity in the tip region and the geometric asymmetry of the tip orifice. This phenomenon may be important in studies of ion transport in charged channels and porous membranes.