Monodisperse Pt nanoparticles with atomic structures that span the cluster to crystal transition have recently been synthesized in electrostatically stabilized, aqueous-based suspensions. In the present study, the anionic charge from the stabilizing SnCl2 sheath adsorbed on the surface of these particles is used for the first time to assemble Pt directly onto porous carbon supports via electrostatic assembly. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals that these assemblies have substantially higher Pt-C dispersions than obtained from precipitation methods commonly used for commercial electrocatalyst systems. Energy dispersive spectroscopy (EDS) and inductively coupled plasma-mass spectrometry (ICP-MS) are used to determine that loadings of 10-30% by weight Pt (particle packing fractions from 0.05 to 0.25) are obtained through a single electrostatic application of these particles on Vulcan carbon, depending on particle size. The highest average oxygen reduction reaction (ORR) mass activity obtained using this approach is 90.4 A/g(Pt) at 0.9 V vs RHE in 0.1 M perchloric acid is with 1-2 run particles that exhibit a transitional atomic structure. This activity compares to an average value of 74.0 A/g(Pt) obtained from densely packed electrostatic layer-by-layer (LbL) assemblies of unsupported particles and 36.7 A/g(Pt) commercial Vulcan electrocatalyst from Tanaka Kikinzoku Kogyo (TICK). Enhanced activity is observed with electrostatic assembly of any particle size on Vulcan relative to unsupported or commercial electrocatalyst with comparable durability. Such enhanced activity is attributed to improved reactant accessibility to the catalyst surface due to the increase in particle dispersion. An extinction coefficient of 7.41 m(2)/g at 352 nm is obtained across the entire cluster to crystal transition from 20 atom clusters to 2.9 nm single crystal nanoparticles, indicating that observed variation in ORR activity with particle size may be associated primarily with changes in atomic surface structure as opposed to the metallic character of the nanoparticles as assessed by UV-vis spectroscopy.