Coating of gold-decorated silica nanoparticles with polyethylene glycol (PEG) was carried out in the aerosol phase. The process involves first functionalizing the nanoparticles at ∼225 ° C with a bifunctional reactant, 2-mercaptoethanol (ME), for which one end is a thiol that attaches to the gold surface and the other end is a terminal hydroxyl group, and then introducing ethylene oxide (EO), which reacts at ∼440 ° C with the hydroxyl group via a ring-opening polymerization to grow PEG. The morphology, elemental composition and surface chemistry of the PEGylated nanoparticles were characterized by means of transmission electron microscopy, energy dispersive x-ray spectroscopy in scanning transmission electron microscopy, x-ray photoelectron microscopy and Fourier transform infrared spectroscopy. The increase in mobility diameter of the nanoparticles due to PEG growth was measured on-line by tandem differential mobility analysis. The PEG coating thickness was found to increase with increases in gold decoration density, flow rate of ME, and flow rate of EO. Coating thicknesses up to ∼4.5 nm were measured on nanoparticles whose initial mobility diameter equaled 39 nm.