Mesoporous carbon aerogel (CA) supported PtCu bimetallic nanoparticles were prepared via a sequential supercritical deposition (SCD) method using supercritical carbon dioxide (scCO(2)). The effects of deposition order of the metal, annealing temperature and metal composition on the average PtCu particle size, size distribution and dispersion were investigated. Four sets of PtCu/CA samples were prepared with two Pt:Cu molar ratios (1:1 and 1:3) and with two different deposition orders (i.e. either Pt or Cu first). X-ray diffraction and electron microscopy data showed that all of the as-prepared samples formed homogeneously distributed disordered PtCu alloy nanoparticles with narrow particle size distributions on the CA support. Increasing annealing temperature in the range 600-950 degrees C increased the average particle size from 1.8 nm to 4.5 nm and resulted in the elimination of separate Cu nanoparticles on the CA surface. The dealloying of the supported PtCu nanoparticles were carried out by cyclic voltammetry and the activity of the dealloyed nanoparticles (after 300 potential cycles) towards the oxygen reduction reaction (ORR) was investigated using rotating disc electrode (RDE) experiments. During dealloying, peaks associated with bulk dissolution and deposition of Cu and dissolution and re-deposition of Cu from the alloyed PtCu nanoparticles were observed at initial cycles along with peaks associated with creation of new Pt sites. Supported nanoparticles with Pt: Cu molar ratios of 1:1 and 1:3 which were prepared by deposition of Cu first had low activities towards ORR after dealloying. On the contrary, nanoparticles prepared by depositing Pt first exhibited promising electrocatalytic activities after dealloying. Samples with a Pt: Cu molar ratio of 1:3 showed higher activities than those with a molar ratio of 1:1. An enhanced ESA of 137 m(2)/g and dealloying induced enhanced mass activity of 0.123 A/mg(Pt) was obtained using the sample with a Pt: Cu molar ratio of 1:3, which was annealed at 800 degrees C. On the other hand, the same sample annealed at 950 degrees C had the highest specific activity of 0.165 mA/cm(2).