Two flat gold electrodes are placed vis-a-vis with an epoxy spacer layer that is etched out to give a ca. 100 m-deep electrochemically active trench. A water-insoluble oil phase, here the redox system N,N-diethyl-NN-didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP), is immobilized into the trench to allow anion transfer upon oxidation of DDPD (oil) to DDPD+ (oil). In mono-potentiostatic mode quantitative transfer/expulsion of anions into the trench oil phase occurs. However, in bi-potentiostatic mode feedback currents dominated by rapid plate-to-plate diffusion normal to the electrode surfaces are observed. Comparison of normal diffusion and lateral diffusion shows that the rate of diffusion-migration charge transport across the oil film is anion hydrophobicity dependent.