A specific technique for measuring fast variations of the high-frequency impedance of ultramicroelectrodes (UME) under potential control is presented. It allows real-time electrolyte-resistance Re measurements from an analogue signal, the amplitude of which is inversely proportional to the modulus of the UME impedance, and from calibration curves measured with pure resistors instead of the electrochemical cell. Re−distance approach curves to insulating or conducting substrates show opposite behaviors:  Re increased when the tip electrode approached an insulating substrate, while it decreased for a conducting substrate, which apparently precludes scanning electrochemical microscopy (SECM) imaging under Re feedback control. Simultaneous current and electrolyte-resistance measurements have been successfully performed in SECM constant-height imaging of various substrates, which allows local information on both topography and surface reactivity to be obtained.