In this communication we elucidate a microstructural picture of proton conduction in nano-crystalline yttria-stabilized zirconia at low temperatures (Kim et al. Adv. Mater., 2008, 20, 556). Based on careful analysis of electrical impedance spectra obtained from samples with grain sizes of approximately 13 and approximately 100 nm under both wet and dry atmospheres over a wide range of temperatures (room temperature-500 degrees C), we were able to identify the pathway for proton conduction in this material. It was found that the grain boundaries in nano-crystalline yttria-stabilized zirconia are highly selective for ion transport, being conductive for proton transport but resistive for oxygen-ion transport.