Further reduction of Pt in hydrogen fuel cells is hampered by reactant transport losses near the catalyst surface, especially for degraded catalysts. Strategically mitigating these performance losses requires an improved understanding of the catalyst nanostructure, which controls local transport and catalyst durability. We apply cryo-tomography in a scanning transmission electron microscope (STEM) to quantify the three-dimensional structure of carbon-supported Pt catalysts and correlate to their electrochemical accessibility. We present results for two carbon supports: Vulcan, a compact support with a large majority of Pt observed on the exterior, and HSC, a porous support with a majority of Pt observed within interior carbon pores, which have relatively constrictive openings. Increasing Pt content shifts the Pt distribution to the exterior on both carbon supports. By correlating to the electrochemical surface area, we find that all Pt surface area is accessible to protons in liquid. However, the interior Pt fraction quantitatively tracks Pt utilization losses at low humidity, indicating that the interior Pt is inaccessible to the proton-conducting ionomer, likely because narrow carbon pore openings block ionomer infiltration. These results imply different proton transport mechanisms for interior and exterior Pt, and quantitatively describe the catalyst structure, supporting development of transport and durability models. (C) The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License.