The transmission electron microscope (TEM) allows us to obtain important information about catalysts at a scale approaching atomic resolution. However, this information is only a 2D projection of the 3D object under investigation. Electron tomography is necessary to obtain spatial information about catalysts. This work aims to apply electron tomography to electrochemical catalysts and to observe the spatial changes of metallic sites of Pt-Co nanoparticles in acidic environments, under elevated temperatures. Four 3D models were created: two for reduced graphene oxide support and two for carbon black support. Electron tomography images were taken on a JEOL JEM-2200FS TEM in the range of 140 degrees. The resulting 3D models with resolutions on the order of a few nanometres evaluate the reduction in catalytic activity. It was found that the choice of support can significantly affect the stability of the whole catalyst; reduced graphene oxide catalysts show greater stability compared to carbon black supported catalysts. The SAXS method was also used as a complementary method to determine the particle size distribution. The 3D electron tomography brings critical insight into nanoscale morphological changes and distinct metal leaching behaviours on different carbon supports.