The effect of solvent isotopic replacement (H for D) on the size of gold nanoparticles (Au NPs) prepared by sodium citrate reduction has been investigated. With increasing replacement of water by deuterium oxide, smaller sizes of Au NPs are obtained, which is interpreted as a consequence of a faster reduction. A mechanism in which a substitution complex, [AuCl 3(C 6H 5O 7) -2] -, is formed from AuCl 4 and citrate ions prior to its rate-limiting disproportionation into products is suggested. This novel procedure offers an attractive alternative to the existing ones and opens a full range of possibilities for biological studies. © 2010 American Chemical Society.