The understanding and control of charge transfer at the very smallest scale is fundamental to nanoscience for applications such as catalysis and energy storage. However, the quantitative measurement of the kinetics of electron transfer at the nanoscale at individual free nanoparticles has not hitherto been possible. Here we describe experiments to unambiguously determine the electron transfer kinetics for the reduction of protons at single gold and silver nanoparticles of radii 7-15 nm. We show that there is a true "nano effect" (a kinetic acceleration due to the size of the nanoparticle) for this reaction at silver nanoparticles, but not at gold nanoparticles. (C) 2012 Elsevier Ltd. All rights reserved.