In the interaction of PVP with Pt nanoparticles < 7 nm in size, charge transfer is from carbonyl groups in PVP to Pt nanoparticles, whereas in the interaction of PVP with bulk Pt or Pt nanoparticles > 25 nm in size, charge transfer is from Pt metal to the polymer side chain of PVP. There exists a critical nanoparticle size between 7 and 25 nm that would lead to a switch in the electron donor-acceptor property.