We demonstrated the self-assembly of atomic Au on thiol-functionalized multiwall carbon nanotubes through covalent bonding and the formation of Au nanoparticles (NPs) upon a subsequent thermal treatment. Au NPs of 3.4 nm were found to exhibit higher ORR activity than that of 1.9 nm, which can be attributed to removal of thiols from Au NP surfaces. This hypothesis is supported by decreasing intrinsic ORR activity with increasing alkanethiol-coverage on polycrystalline-Au. The understanding on top of the novel synthetic route has led to the development of Au NPs less than 5 nm with the highest ORR activity reported to date.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR–0819762)United States. Dept. of Energy (DE-AC02-98CH10886)Brookhaven National LaboratorySamsung Foundation of Culture (Korea) (Scholarship)