This investigation focuses on the aggregation behaviour of Pt catalyst nanoparticles in PEMFCs. Here, Pt nanoparticles were prepared using various surfactants which are known to prevent aggregation. Dispersed nanoparticles are thought to have a higher available electrochemical surface area, hence exhibiting higher catalytic activity than aggregated nanoparticles. In our study, Pt nanoparticles were prepared in aqueous dispersion using tetradecyltrimethylammonium bromide (C 14TAB), cetyltrimethylammonium bromide (C 16TAB) and nonylphenolethoxylate (NP9). The aggregation behaviour of the nanoparticles was studied using Transmission Electron Microscopy (TEM), Nanosight and Dynamic Light Scattering (DLS). Electrochemical experiments on Pt Rotating Disc Electrode (RDE) in the absence and presence of surfactants at 298 K show that the surfactants have a marked effect on the obtained linear sweep voltammograms (LSVs). The movement of the onset potential to lower overpotentials suggests that the surfactant is 'poisoning' catalytic sites on the electrode. This also demonstrates that the surfactant selection is vital to obtaining effective PEMFC cathode catalyst.