We report a method to control the size of platinum nanoparticles synthesized by cathodic corrosion. Using a 5 M solution of NaOH we show that current density during the synthesis determines the size of the resulting nanoparticles, producing an almost linear dependence for particle sizes between 6.5 and 12.5 nm in diameter. The catalytic electro-oxidation of carbon monoxide on these cathodic nanoparticles shows that not only the size, but possibly also their surface termination can be controlled. While smaller nanoparticles appear poly-oriented, larger ones exhibit CO oxidation activity characteristic of extended Pt(1 0 0) terraces. Comparing the results obtained in NaOH and KOH we conclude that the depletion in the diffusion layer close to the electrode is the leading factor influencing the nanoparticle size, and suggest further ways to control this new method of cathodic synthesis of nanoparticles. (C) 2013 Elsevier Ltd. All rights reserved.