A　water-assisted　control　of　Pt　nanoparticle　size　during　a　surfactant-free,　microwave-assisted　polyol　synthesis　of　the　carbon-supported　platinum　nanoparticles　(Pt/C)　in　a　mixture　of　ethylene　glycol　and　water　using　(NH4)(2)　PtCl6　as　the　Pt　precursor　is　demonstrated.　The　particle　size　was　tuned　between　similar　to　2　and　similar　to　6　nm　by　varying　either　the　H2O　volume　percent　or　the　Pt　precursor　concentration　during　synthesis.　The　electrochemical　surface　area　(ECSA)　and　the　oxygen-reduction　reaction　activity　obtained　for　the　Pt/C　electrocatalyst　show　a　catalytic　performance　competitive　to　that　of　the　state-of-the-art　commercial　Pt/C　electrocatalysts　used　for　polymer　electrolyte　membrane　fuel　cell　electrodes　(ECSA:　similar　to　70　m(2)/g;　half-wave　potential　for　oxygen　reduction　reaction:　0.83　V　vs　reversible　hydrogen　electrode).　The　synthesized　Pt/C　electrocatalysts　show　durability　equivalent　to　or　better　than　that　of　the　commercial　Pt/C.　The　durability　was　found　to　improve　with　increasing　particle　size,　with　the　ECSA　loss　values　being　similar　to　70　and　similar　to　55%　for　the　particle　sizes　of　2.1　and　4.3　nm,　respectively.　The　study　may　be　used　as　a　route　to　synthesize　Pt/C　electrocatalysts　from　a　convenient　and　economic　Pt　precursor　(NH4)(2)　PtCl6　and　avoiding　the　use　of　alkaline　media.