The width δ of the active electrode area along the triple phase boundary, TPB, is determined using a novel method based on ac impedance measurements between two interdigitated electrodes. The theoretical basis is presented and used to derive the width δ for gold electrodes applied onto the solid electrolyte (SE) Y2O3 stabilized ZrO2 (YSZ). Gold electrodes are used in the demonstration of the method due to their ion blocking nature and poor cathodic properties which requires that all or at least most of the electrochemical reaction steps take place on the free surface of the SE near the TPB. The measurements are done at relatively low temperatures of 250–300 °C. The width δ is determined to be 0.4, 0.3 and 0.2 μm at 250, 275 and 300 °C, respectively, decreasing with increasing temperature. In addition to the width δ, the ionic conductivity of YSZ at those low temperatures is determined as 2.2 × 10− 7, 7.2 × 10− 7 and 2.3 × 10− 6 S/cm at 250, 275 and 300 °C, respectively, with an activation energy of 1.27 eV.