A method for the in situ preparation of Pt nanoparticles of controlled size based on the modification of highly oriented pyrolytic graphite (HOPG) with aminophenyl groups employing diazonium chemistry is demonstrated. It is shown that full chemical reduction of an Ar–NO2 grafted surface is achieved with aqueous Sn(II). The formation of aminophenyl groups was established by chemical reaction with 1,2-benzoquinone followed by electrochemical detection of the bound quinone using square wave voltammetry. The oxidation state of nitrogen was confirmed by XPS. Platinum nanoparticles were prepared chemically by the attachment of Pt(II) on the Ar–NH2 modified electrodes followed by reduction with NaBH4. This resulted in the formation of Pt nuclei that could be further grown. The surfaces formed were characterized by TEM, XRD and XPS. The dependence on particle size of the electro-oxidation of CO and methanol in 0.5 M H2SO4 was studied for three different nanoparticle sizes, 2.0, 2.7 and 4.0 nm. It is demonstrated that reactivity for these reactions decreases with decreasing particle size. The difficulties in comparing particle sizes calculated from XRD data with those from TEM are discussed.