Stable gold nanoparticles of 2-nm diameter are prepared in water by chemical reduction of AuCl4- by NaBH4 in the presence of CS2 without adding any other stabilizers. The nanoparticles are proved to be Au and not Au sulfides by electron diffraction.. In their UV-visible absorption spectra the surface plasmon (SP) band at around 520 nm is strongly damped, due to small particle size and possibly chemisorption of a CS2 derivative. The time course of UV-vis reveals a rapid decrease of the 290-nm CT band of AuCl4- upon addition of CS2, suggesting their interaction before reduction of Au3+. After the reduction of Au3+ has been completed, an intense band is built up at 330 nm which can be assigned to a reaction product of CS2 and NaBH4. Furthermore, it is found that this reaction is promoted by Au nanoparticles, although no change was observed for the SP band intensity. This catalytic effect is also found forcitrate-capped Au hydrosols.