The layer-by-layer (LbL) desposition of oppositely charged polyelectrolytes from adsorption solutions of different ionic strength onto ∼7 nm diameter carboxylic acid-derivatized gold nanoparticles has been studied. The polyelectrolyte-modified nanoparticles were characterized by UV-vis spectrophotometry, microelectrophoresis, analytical ultracentrifugation, and transmission electron microscopy. UV-vis data showed that the peak plasmon absorption wavelength of the gold nanoparticles red-shifted after each adsorption step, and microelectrophoresis experiments revealed a reversal in the surface charge of the nanoparticles following deposition of each layer. These data are consistent with the formation of polyelectrolyte layers on the nanoparticles. Analytical ultracentrifugation showed an increase in mean nanoparticle diameter on adsorption of the polyelectrolytes, confirming the formation of gold-core/polyelectrolyte-shell nanoparticles. Transmission electron microscopy studies showed no signs of aggregation of the polyelectrolyte-coated nanoparticles. The adsorption of the polyelectrolyte-coated gold nanoparticles onto oppositely charged planar supports has also been examined. UV-vis spectrophotometry and atomic force microscopy showed increased amounts of nanoparticles were adsorbed with increasing ionic strength of the nanoparticle dispersions. This allows control of the nanoparticle surface loading by varying the salt content in the nanoparticle dispersions used for adsorption. The LbL strategy used in this work is expected to be applicable to other nanoparticles (e.g., semiconductors, phosphors), thus providing a facile means for their controlled surface modification through polyelectrolyte nanolayering. Such nanoparticles are envisaged to have applications in the biomedical and bioanalytical fields, and to be useful building blocks for the creation of advanced nanoparticle-based films.