In order to improve the performance of solid oxide fuel cells (SOFCs), there is a need to deepen our understanding of the cathode reactions, which is rate-limiting. The purpose of this work is to develop and demonstrate a direct electrochemical technique for estimating the cathode triple-phase boundary (TPB) between electrode, electrolyte and gas phase. Interdigitated gold micro-electrodes are fabricated on YSZ for this purpose as gold is oxygen-blocking, forcing the oxygen to enter into the solid electrolyte at the TPB. Three-electrode measurements are performed, allowing ac impedance measurement between two interdigitated gold electrodes while at the same time driving dc current between one of the gold electrodes and a silver counter electrode. Impedance spectroscopy (IS) measurements are performed at 300 °C. IS results demonstrate two main features — a large low-frequency (LF) arc and a smaller high-frequency (HF) arc, which actually consists of two overlapping arcs. The results suggest that the HF arc reflects mainly the surface impedance between the opposing TPBs and that the TPB width is much less than 8 μm and estimated to be at most 1 μm. The TPB width increases under high anodic polarization. The LF arc reflects the electrode impedance.