A relationship between the tip response and the tip-to-substrate separation distance was investigated under alternating current scanning electrochemical microscopy (AC-SECM) conditions. For insulating substrates only negative AC feedback was observed. However, in the case of conductive substrates positive as well as negative AC feedback could be observed depending on experimental conditions. The theoretical model proposed in this work explains very well all observed trends and allows for qualitative predictions of the tip impedance in AC-SECM as a function of various experimental conditions (including: frequency, the supporting electrolyte concentration, the tip-to-sample separation distance and the tip diameter). In this theoretical model the double-layer capacitance of the tip and the sample is represented by a constant phase element (CPE), rather than an ideal capacitor. The paper also describes how raw experimental impedance data can be corrected for imperfections of the measuring system. (c) 2006 Elsevier Ltd. All rights reserved.